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A Guide to John Locke's Essay concerning Human Understanding

A Guide to John Locke's Essay
concerning Human Understanding

John Locke's An Essay Concerning Human Understanding is a classic statement of empiricist epistemology. Written in a straightforward, uncomplicated style, the Essay attempts nothing less than a fundamental account of human knowledge—its origin in our ideas and application to our lives, its methodical progress and inescapable limitations. Even three centuries later, Locke's patient, insightful, and honest reflections on these issues continue to merit the careful study that this guide is intended to encourage.

Locke prefaced his masterwork with a rhetorically understated "Epistle to the Reader." His awareness of the need for a systematic investigation of the human understanding first arose in the context of a friendly but unproductive discussion of other issues. (According to another of the participants in that meeting, they included "the Principles of morality, and reveald Religion.") Although he drafted a preliminary account that dealt with many of the central themes of the Essay as early as 1671, Locke expanded his comments repeatedly before publishing the book nearly twenty years later and continued to supplement them with additional material he prepared for four further editions. Claiming only to be an "Under-Labourer" whose task is to prepare the way for the "Master-Builders" of science, he encouraged ordinary readers to rely upon their own capacity for judgment rather than to accept the dictates of intellectual fashion. [Essay Epistle]

In the daily course of ordinary activity, everyone is inclined to rely upon a set of simple guidelines for living, and laziness or pride may encourage us to accept dearly held convictions without ever embarking on a careful examination of their truth. But this is a dangerous course. Locke pointed out that blind acceptance of "borrowed Principles"—the confident pronouncements of putative cultural authorities regarding crucial elements of human life—often leaves us vulnerable to their imposition of absurd doctrines under the guise of an innate divine inscription. [Essay I iii 24-26] Our best defense against this fate is to engage in independent thinking, which properly begins with a careful examinination of the function and limits of our discursive capacities.

Attention to specific issues at hand often leads us to overlook the function of the most noble of our faculties, but Locke believed that the operations of the human understanding are familiar to us all. We employ ourselves in thinking, deciding, doing, and knowing all the time. What we require is not a detailed scientific explanation of the nature of the human mind, but rather a functional account of its operations in practice. For that purpose, Locke supposed, we must pursue the "Historical, plain Method" of observing ourselves in the process of thinking and acting. With respect to each significant area of human knowledge, we must ask ourselves: where does it come from, how reliable is it, and how broadly does it extend? [Essay I i 1-2]

The last of these questions is arguably most to the point. Locke realized early on in his epistemological reflections that skeptical doubts often arise from unreasonable expectations about the degree of certainty it is possible for us to attain. [King, p. 107] Academic philosophers have contributed to the problem by demanding demonstrative certainty of the speculative truth even in instances where we are unlikely to be able to achieve it. But their demands for excessive precision in philosophical language lead only to pointless wrangling over the meanings of their terms, on Locke's view. The simple truth is that we can't be certain about everything, and it would be counter-productive to try to expand our knowledge beyond its natural limits. Since we are not capable of knowing everything, contentment with our condition requires a willingness not to reach beyond the limitations of our cognitive capacities. Our intellectual energy would be most efficiently employed were we to avoid intractable disputes over matters beyond our ken and rely instead upon our "Satisfaction in a quiet and secure Possession of Truths, that most concern'd us." [Essay I i 7] In ordinary life, we know what we need to know, and expecting more than that would only lead us to despair.

After all, Locke argued, we do have what we need most. The practical conduct of human life doesn't depend upon achieving speculative certainty about the inner workings of the natural world or acquiring detailed information about our own natures. It will be enough if we can secure "the Conveniences of Life" and recognize what we ought to do. How short soever their Knowledge may come of an universal, or perfect Comprehension of whatsoever is, it yet secures their great Concernments, that they have Light enough to lead them to the Knowledge of their Maker, and the sight of their own Duties. Men may find Matter sufficient to busy their Heads, and employ their Hands with Variety, Delight, and Satisfaction; if they will not boldly quarrel with their own Constitution, and throw away the Blessings their Hands are fill'd with, because they are not big enough to grasp every thing. [Essay I i 5] The pursuit of happiness—the genuine business of human life, on Locke's view—demands only that religion, morality, and science be established to a degree that permits practical progress.

Even with respect to such vital matters, Locke supposed, our knowledge is often limited. The testimony of our senses, together with a natural inclination to seek pleasure and avoid pain, guides much of our daily conduct even though sensitive knowledge cannot offer demonstrative certainty about the existence of an external world. [Essay IV xi 8-10] It would be foolish indeed to refuse to eat simply because God has not granted us speculative certainty regarding the nutritional efficacy of food. Divine provision for the practical needs of human life is expressed more economically: So in the greatest part of our Concernment, he has afforded us only the twilight, as I may so say, of Probability, suitable, I presume, to that State of Mediocrity and Probationership, he has been pleased to place us in here. [Essay IV xiv 2] The pace of ordinary life commonly requires all of us to act quickly, on the basis of little more than our fallible memory of the merely probable evidence in favor of this course of action, Locke supposed, but awareness of this common reliance on "our mutual Ignorance" appropriately motivates broad toleration of the diversity of opinions and practices to which others may adhere. [Essay IV xvi 3-4]

The great theme of the Essay, then, is that we ought not to expect to achieve knowledge beyond the relatively narrow confines of what is necessary or, at least, useful for the practical conduct of human life. Although speculative knowledge of the essences of God, human beings, and material things exceeds the capacity of our cognitive faculties, according to Locke, we have no grounds for complaint. Observation of the causal regularities in nature enables us to secure our survival and comfort, "ease, safety, and delight," during this life. What is more, evaluation of our moral conduct in the light of our accountability to God for the actions we perform provides amply for our hope of a better existence beyond this life. Limited though it may be, Locke supposed, the human capacity for knowledge is sufficient for our happiness here and hereafter, and since that is that is our primary concern, it would be pointless to demand that our faculties reach any further. [King, pp. 86-92]

A year before the first edition was published, Locke wrote an "Abstract of the Essay." Translated into French by his friend Jean le Clerc, this document was published in the Bibliothèque Universelle in 1688, giving the European intellectual community a full preview of the work to come. This presentation of the central themes indicates what Locke himself regarded as his most significant contributions to the subject.

After a mere mention of the polemic against innate ideas, Locke explained his own belief that all human thought originates in the simple ideas of sensation and reflection. He devoted particular attention to the primary/secondary quality distinction and to the acquisition of simple ideas of space, time, number, pleasure, and pain. Then he outlined the account of our formation of crucial complex ideas, including those of substances, mixed modes, and relations. [King, pp. 365-378]

Noting his own belated discovery of the vital importance of language, Locke offered a basic statement of his own theory of language, with special attention to the relation between general terms and abstract ideas. Drawing the distinction between civil and philosophical uses of language, he pointed out that difficulties in communication result both from the natural imperfections of language and from its deliberate misuse. [King, pp. 378-388]

Finally, Locke defined knowledge and distinguished its several types, each of which is subject to strict limitations. Arguing in some detail against the common inclination to rely upon supposedly self-evident principles, Locke proposed that genuine advances in human knowledge depend instead upon the proper exercise of good judgment in assenting to opinions suitable to the ideas with which they are concerned. [King, pp. 388-398]

Locke rarely commented explicitly on the relation of his own work with that of other thinkers. Sharing the wide-spread seventeenth-century scorn for sectarian proponents of scholastic philosophy, he warned that reliance upon supposed authorities—ancient or modern—more commonly hinders than helps the pursuit of truth. [Conduct 24] Nevertheless, Locke wrote with an extensive and thorough knowledge of the philosophical tradition.

Though he decried the evils of the slavish "Aristoteleans," Locke himself often relied upon the writings of Aristotle, especially in matters of logic and ethics. The influence of classical Stoicism, especially as represented by Cicero, is also evident at many points in the Essay. Medieval philosophers and theologians seem to have made rather less of an impression on him, though it is likely he would have known the work of Augustine, Aquinas, and Ockham. Locke expressed great respect for Francis Bacon, on whose discussions of language and logic he relied, but often criticized Thomas Hobbes, whose analyses of volition and political organization clearly influenced his own thinking.

Locke's relationship to the French Cartesians is more complex. Although he obviously respected the fresh initiatives René Descartes had brought to seventeenth-century philosophy, Locke had serious reservations about the success of the Cartesian approach. He explicitly criticized its identification of body with space under the attribute of extension and its emphasis on the ontological argument for God's existence. Although he sometimes presupposed a dualistic account of human nature, Locke disputed many of its Cartesian corrolaries, including the continual thinking of the soul and the absence of thought in animals, and he notoriously suggested the possibility that matter might have the power to think. In general, Locke disavowed the over-reliance on mathematical reasoning at the expense of sensory observation in the pursuit of human knowledge. His letters include several critical examinations of Gassendi and Malebranche as well.

Among his contemporaries, Locke more clearly admired scientists than philosophers. Although he knew many of the Cambridge Platonists, Locke shared few of their convictions, and his attack on innatism may well have been addressed against them. From his friend Robert Boyle he gleaned the distinction between primary and secondary qualities, along with a profound appreciation for the corpuscularian theory of matter. Of course he reserved high praise for the achievements of Isaac Newton, though Locke's grasp of Newtonian mathematics often seems rather shaky, and the letters the two exchanged were most frequently concerned with biblical interpretation. Locke's own training in the medical sciences did occasionally provide ready examples for points under discussion in the Essay.

Despite the modesty with which he offered his epistemological reflections, Locke showed great interest in the public reception they received. Publication of the "Abstract" drew ample attention even before the book itself had appeared, and from that time on, Locke's correspondence was full of suggestions and complaints from other thinkers. Even when he later modified his views, however, Locke rarely acknowledged this reflected the influence of such criticism, which he tended to regard as unjustified attacks borne of malicious mis-reading. He persistently declined to engage in a direct correspondence with Leibniz; only the hapless Bishop Stillingfleet succeeded in drawing him into a public debate. On the whole, there is little evidence of the openness to critical reflection Locke frequently expressed in his letters to friends like Molyneux, le Clerc, Tyrrell, and Burnett.

Locke used the word "idea" for the most basic unit of human thought, subsuming under this term every kind of mental content from concrete sensory impressions to abstract intellectual concepts. Explicitly disavowing the technical terms employed by other philosophical traditions, he preferred simply to define the idea as "whatsover is the Object of the Understanding when a Man thinks." [Essay I i 8] Locke worried little about the ontological status of ideas. He did commonly refer to them as being "in the Mind," both when we are conscious of them and when they are stored in memory, he regarded this as no more than a spatial metaphor. Locke was interested in these immediate objects of perception only because they point beyond themselves.

Thus, the crucial feature of ideas for Locke was not what they are but rather what they do, and the epistemic function of an idea is to represent something else. For since the Things, the Mind contemplates, are none of them, besides it self, present to the Understanding, 'tis necessary that something else, as a Sign or Representation of the thing it considers, should be present to it: And these are Ideas. [Essay IV xxi 4] Because we do think and must always be thinking about something or other, then, it follows that we actually do possess ideas. [Essay II i 1] If we want to comprehend the foundations for human knowledge, Locke supposed, it is natural to begin by investigating the origins of its content.

Since knowledge—indeed, human thought of any sort—is mediated by ideas, it is well worth asking how we acquire them. Thus, in Book II of the Essay, Locke embarked on an extended effort to show where we get all of the ideas that we do so obviously possess. An adequate genetic account will explain, at least in principle, how human beings acquire the ability to think about anything and everything. Let us then suppose the Mind to be, as we say, white Paper, void of all Characters, without any Ideas; How comes it to be furnished? Whence comes it by that vast store, which the busy and boundless Fancy of Man has painted on it, with an almost endless variety? Whence has it all the materials of Reason and Knowledge? To this I answer, in one word, From Experience: In that, all our Knowledge is founded; and from that it ultimately derives it self. [Essay II i 2] The human mind is like a camera obscura for Locke, a darkened room into which bright pictures of what lies outside must be conveyed. [Essay II xi 17]

Locke had already argued at length that ideas are not innately imprinted on the human mind. Observing children reveals that their capacity to think develops only gradually, as its necessary components are acquired one by one. No individual idea is invariably present in every human being, as one would expect of an innate feature of human nature, and even if there were such cases, they could result from a universally-shared experience. Everything that occurs to us either arrives directly through experience, or is remembered from some previous experience, or has been manufactured from the raw materials provided solely by experience. [Essay I iv]

From the outset of the project, then, Locke took the empiricist stance that the content of all human knowledge is ultimately derived from experience. We can only think about things we're acquainted with in one or the other of two distinct ways: Our Observation employ'd either about external, sensible Objects; or about the internal Operations of our Minds, perceived and reflected on by our selves, is that, which supplies our Understandings with all the materials of thinking. These two are the Fountains of Knowledge, from whence all the Ideas we have, or can naturally have, do spring. [Essay II i 2] Notice that Locke distinguished sensation and reflection by reference to their objects. We acquire ideas of sensation through the causal operation of external objects on our sensory organs, and ideas of reflection through the "internal Sense" that is awareness of our own intellectual operations. As the rest of Book II is designed to show, these two sources provide us with all of the ideas we can ever have. [Essay II i 3-5]

The acquisition of ideas is a gradual process, of course. Newborn infants, Locke supposed, are first aware of the vivid experiences of their own hunger or pain. Then, by further experience, they acquire a supply of sensory ideas from which they can abstract, learning to distinguish among familiar things. Only later do they attend to their reflective experience of mental operations in order to acquire ideas of reflection. [Essay II i 21-24] Since we come to have ideas only by means of our own experience, Locke supposed, any interruption of this normal process could prevent us from having them. Having defective organs of sense, artificially restricting experience, or inattentively observing what we have can all limit our possession of mental contents. [Essay II i 5-8] Individual human beings therefore exhibit great differences in their possession of simple ideas, and Locke speculated that other sentient beings—having, for all we know, experiences very different from our own—are likely to form ideas of which we can have no notion at all. Since simple ideas are acquired only by experience, anything we do not experience is literally inconceivable to us. [Essay II ii 1-3]

Everything begins, then, with simple ideas of sensation. Most of these are uniquely produced in the mind through the normal operation of just one of the organs of sense. Our ideas of colors, sounds, smells, tastes, and heat, Locke supposed, are acquired respectively through our eyes, ears, noses, tongues, and skin. Lacking the appropriate organ in any of these cases would wholly prevent our having any of the characteristic ideas of that sense. With normal sensory organs, we come to have so many simple ideas of sensation that we don't bother to invent words naming all of them. [Essay II iii 1-2] Notice that these ideas tend to be either the unnamed determinate instances of some determinable predicate (particular shades of blue and varieties of sourness) or sensations easily identifiable by association with other ideas (the taste of pineapple and the fragrance of a rose).

According to Locke, certain special simple ideas are acquired by two different senses. Space, extension, figure, motion, and rest are all presented to us both in sight and in touch; they are therefore among the most commonly received of all our ideas of sensation. [Essay II v] Because of their prominence, specific ideas of these kinds constitute the basis for the most fundamental organization of our sensory experience. All of them represent primary qualities of sensible objects and serve significant roles in science and ordinary life. Things that can be both seen and touched seem most obviously real to us.

But is it correct to suppose that one and the same idea can be acquired from either of two distinct senses? Since simple ideas of sensation cannot be acquired through defective sensory organs, on Locke's view, it should be impossible to acquire the visual idea of motion from tactile sources alone. Locke's Irish friend William Molyneux posed this problem with precision in letters to the Bibliothèque Universelle and to Locke himself. [Correspondence 1064, 1609, 1620.] Supposing that someone blind from birth became familiar with solid figures by touch alone and then later gained the power to see, would this person be able to distinguish a cube and a sphere by sight alone without first touching them? In a passage added to the Essay's second edition, Locke agreed with Molyneux (on a view confirmed by twentieth-century empirical research) that the answer must be no. [Essay II ix 8] The visual and tactile ideas of the globe are distinct. Although we use the same words to designate ideas of shape and motion whether they have come from sight or from touch, only "an habitual custom" associates ideas from distinct senses with each other.

In a few, even more special instances, simple ideas are produced in us by reflection as well as sensation. These are ideas that are invariably present in the mind in association with every other object of thought, no matter what its source. According to Locke, such ideas of both sensation and reflection include pleasure, pain, power, existence, and unity. [Essay II vii] Among these existence and unity secure yet another aspect of the organization of our experiences, by providing clear conceptions of reality. Pleasure and pain, as we'll see later, play a special role in motivating us to exercise the volitional power behind all human actions, of mind and body.

Although Locke assumed that simple ideas of sensation are invariably produced in our minds by the influence of external objects on our organs of sense, he did not suppose this causal process to be straightforwardly representational: there need not always be something in the object that corresponds directly with the idea it produces in us. In some cases, for instance, it is the absence, rather than the presence, of some determinate feature in the thing that produces "a real positive Idea in the Understanding." [Essay II viii 1-2] Even on a causal theory of perception, privation is a change in the sensory organ, and therefore fully perceptible. Thus, our ideas of cold, darkness, and (perhaps) rest are produced by the absence of heat, light, and motion in things.

Despite the systematic ambiguity of our usual vocabulary, Locke maintained, we should always distinguish between ideas themselves, the immediate objects of thought as entertained by the mind, and qualities, the causal powers by means of which external objects produce those ideas in us. [Essay II viii 7-8] An examination of the causal processes involved in formation of our sensory ideas thusly led Locke to propose an important distinction among the qualities of bodies:

The primary qualities of any body are features Locke took to be inseparable from it, both in the sense that they are the kinds of features we experience to be aspects of every body and in the sense that we suppose that they would remain present even if the body were divided into imperceptible parts. [Essay II viii 9] Our ideas of the primary qualities include those of solidity, extension, figure, motion, rest, and number. Notice that all of these ideas are perceived by touch and that all except solidity are perceivable by sight as well. Since bodies operate only on mechanistic principles, our perception of such ideas can only result from a direct contact between our organs of sense and the object itself—or, in the case of vision at a distance, an imperceptible particle from the object itself. [Essay II viii 12] Such qualities therefore exist as features of the body itself, independently of their perception.

Secondary qualities, on the other hand, Locke held to be nothing in the object itself except a causal power to produce ideas of a certain sort—a color, a taste, or a sound, for example—in us. Like the mere powers of producing change in other objects, these supposed qualities are nothing other than effects produced by the genuine (primary) qualities of bodies themselves. [Essay II viii 10] Notice that all of our ideas of secondary qualities are produced in us only by a single sense; their perception clearly depends upon the normal operation of our sensory apparatus in response to the merely dispositional properties of things. Thus, our customary supposition that such qualities really exist in nature is nothing more than a reification of the ideas that are actually produced in our minds by the primary qualities of things. [Essay II viii 14-17]

Locke's arguments in defence of this thesis may be familiar from their prior statement in Galileo, Descartes, and Boyle or from their later appropriation by Berkeley and Hume. The color and taste of manna (a natural laxative) are less like its powdery texture than like its effect on the digestive tract; although things themselves remain the same in darkness as in light, their color appears only through the mediation of rays of light; contrapositively, the mechanical process of grinding an almond can change only its primary qualities, yet the secondary qualities are thereby modified; and a single body of tepid water can seem both warm and cool at the same time, if our fingers have been differentially chilled and heated beforehand. [Essay II viii 18-21] In every case, the proper conclusion is that the ideas of secondary qualities are produced in us only by the mechanical operation of the primary qualities of bodies.

From this distinction, Locke concluded that there is a significant difference in the representational reliability of primary and secondary qualities. Our ideas of primary qualities resemble the qualties themselves; the mind-independent features of material objects are (at least at the microscopic level) exactly as we perceive them to be. But our ideas of secondary qualities and powers resemble nothing in the material world; they are the causal consequences, in our minds, of the mechanical action upon our sensory organs of corpuscular primary qualities. [Essay II viii 23-24] As his Royal Society companion Robert Boyle had shown, the rapid motion of insensible particles of matter is a genuine feature of objects, a primary quality that can be transmitted mechanistically from one body to another: as a result, fire has the power to melt other bodies and—in the same way—to produce in us the ideas of warmth, heat, or even pain. [Essay II viii 15-16]

Although he insisted that human awareness begins with simple ideas of sensation, Locke did not believe that they comprise all of our mental contents. We also acquire simple ideas of reflection through an objective observation of our own mental operations. In any these operations, the human mind must be either passive or active, Locke supposed, and the most fundamental ideas of reflection are therefore just two in number: perception, in which the mind passively receives ideas, and volition, in which it actively initiates something. [Essay II vi 1-2] Every other idea of reflection, on Locke's view, is a simple mode of one or the other of these two basic types. All human mental activities derive from the faculties of understanding and will.

We are naturally familiar with such activities, since they constitute the whole of our conscious thought, but this does not entail our having clear conceptions of them. Familiarity without careful attention provides only confused ideas of our own mental operations. As Locke had already noted, the components of experience we first acquire are vivid sensory impressions of the external world. It is only with increasing maturity and a capacity for detachment that we grow able to make the careful inward observations from which clear ideas of reflection may arise. [Essay II i 7-9] Even when we have acquired clear ideas from reflection, Locke supposed, we often designate them with a vocabulary drawn from the concrete context of sensory experience, representing our "inner" mental operations by an implicit reference to observable "outer" processes. [Essay III i 5]

The first and simplest of our ideas of reflection is that of perception, the passive reception of ideas through the bodily impressions made by external objects upon the organs of sense. Although perception in this sense is distinguished from thinking generally by the relatively meager degree to which it falls under our voluntary control, Locke believed it always to require some degree of conscious attention. Physical stimulation of the sensory organs does not produce ideas unless the mind is attentive. [Essay II ix 1-4] Here Locke tilts to a significant feature of our sensory experience. Although we are almost constantly bombarded by physical stimuli—any one of which will, in the ordinary course of things, produce in our minds a particular sensory idea—we never perceive all of them and sometimes exercise deliberate control over which ones we do perceive. Attention, choice, and judgment can all influence the operation of the officially passive faculty of perception. [Essay II ix 7-8]

Another of our cognitive capacities is the ability to retain ideas in the mind over time, either in continuous contemplation or in recollection of ideas after a period of inattention. Although he naturally relied upon the spatial metaphor of memory as a "Store-house" or "Repository" of ideas, Locke emphasized that the ideas we can recall are not actually in the mind—that is, we are not conscious of them at all—during the inattentive interval. Memory is just the power to bring to mind ideas that we are aware of having perceived before without simply perceiving them anew. [Essay II x 1-2] Like Descartes, Locke assumed that the proper function of human memory relies upon some physical process within the human body, as shown by the gradual effacement of infrequently experienced ideas and the loss of memory as a result of bodily disease. [Essay II x 4-5] On the other hand, some ideas are more firmly and easily retained than are others. Greater degrees of attention and frequent repetition of experience have some influence on this, but Locke supposed that the most important factor is the association of certain of our ideas with pleasure and pain. The retention of ideas thusly correlated with the achievement or loss of happiness is one of the benevolent provisions for the needs of human life, since it motivates us to avoid or prefer experiences of appropriate sorts. [Essay II x 3] Being able to remember our past experiences of hunger or headache without actually feeling their pain, he argued, encourages us to act in such ways as are likely to forestall their recurrence. [Essay IV xi 5-6]

Our reflective ideas of other mental operations receive much shorter treatment. Discerning is the mental operation of distinguishing among our ideas--either quickly by wit or carefully in sober judgment. This is a vital tool in providing the supply of distinct ideas whose interrelations we intuitively grasp in assenting to self-evident truths. [Essay II xi 1-3] In the same fashion, the activity of comparing gives rise to our knowledge of relations by noticing non-exact similarities among our ideas. [Essay II xi 4-5] Compounding is the mental capacity to manufacture new complex ideas out of relatively simple components, while abstracting is the ability to make general use of particular ideas in more general reference by stripping away their indexical features. These processes are especially vital because they ground our linguistic competence in employing general terms and secure the possibility of universal knowledge. [Essay II xi 6-9] Distinguished by their phenomenal quality, other cognitive powers are comprehended by the reflective ideas of sensation, remembrance, recollection, contemplation, reverie, attention, study, dreaming, and ecstasy. [Essay II xix 1-2]

Although each human being must acquire the ideas of reflection by observation of her own mental operations, on Locke's view, the activities thusly conceived may then be attributed to other sentient beings, qualified with the suspicion that they may vary greatly in degree from case to case. [Essay III vi 11] There is ample evidence of individual variations even among similarly-constituted fellow humans, and it is reasonable to suppose that non-human thinkers could differ in ways that we cannot literally imagine. Although our own faculty of perception is perfectly suited to the conduct of human life, for example, Locke noted explicitly that other spirits, whose embodiment involves sensory organs different from our own, might well perceive aspects of the world of which we are unaware. [Essay II xxiii 12-13] Dependent as we are on a sense of sight with a particular acuity, we can only speculate how the natural world might appear to beings whose visual abilities were different, much less those for whom the senses of smell or hearing are more fundamental.

Although Descartes and his followers had dogmatically argued that non-human animals, lacking possession of a separable thinking substance or soul, must therefore be incapable of thinking in any form, Locke preferred to examine the empirical evidence in an effort to discover to what degree animals may be capable of engaging in mental activity of each sort. [Essay II i 19] The intellecual capacity for perception, Locke held, marks the distinction between animal and plant life, and the sensory faculties of each species are well-suited to the practical needs of life for animals of that sort. Since individual human beings differ widely in their intellectual abilities, Locke even speculated that those of some animals may exceed those of the least gifted of human thinkers. [Essay II ix 11-15] In the case of the retention of ideas, he suggested that the behavior of songbirds clearly exhibits their deliberate effort to learn a new tune. [Essay II x 10]

Nevertheless, Locke believed that the higher intellectual abilities of animals are strictly limited. They do not compare or discern ideas that lie beyond their immediate "sensible Circumstances," nor are they capable of compounding the new complex ideas required for engaging in mathematical thought. [Essay II xi 5-7] Most notably, he maintained that non-human animals are utterly incapable of abstraction. Even if their sensory capacities permit them to engage in some degrees of intuitive thinking, then, their inability to form abstract ideas and employ general terms make it impossible for them to rise above particularity in order to engage in moral reasoning. [Essay II xi 10-11] Thus, Locke preserved a significant moral distinction between human beings and other animals without grounding it on an ontological distinction in the possession of an immaterial soul.

Although he granted that some groups of simple ideas naturally occur together in our experience, Locke supposed that most of our complex ideas are manufactured in the human mind by the application of its higher powers. Combining joins several simple ideas together in the formation of a new whole; comparing brings two distinct ideas together without uniting them, giving rise to the idea of a relation between them; and abstracting separates some aspect of an idea from its specific circumstances in order to form a new general idea. Repeated applications of these powers, Locke supposed, give rise to the whole variety of ideas human beings are capable of having. [Essay II xii 1-2] If we're going to analyze the epistemic origins our complex ideas, it will be helpful to consider the ways in which we gradually build up our supply of them.

On Locke's view, complex ideas are of three varieties: Modes are invariably conceived as the features of something else, which are never capable of existing independently. Substances, on the other hand, are understood to be the existing things in which modes inhere. Relations are nothing more than mental comparisons in some respect among other ideas. [Essay II xii 3-7] Complex ideas of all three sorts are manufactured by the mind from simpler components.

A simple mode is a complex idea all of whose component parts are variations or combinations of a single simple idea. [Essay II xii 4-5] Consider, for example, the simple idea of space: acquired initially from our senses of sight and touch, this idea provides the sole content for a host of related ideas of our own manufacture. The notion of one-dimensional length can be "folded" upon itself to yield those of area in two dimensions and capacity in three; these notions, in turn, can be modified more subtly to provide our ideas of shapes and figures. Even the notion of place in relation to other bodies within a framework Locke believed to be abstracted from the simple idea of space. [Essay II xiii 2-6] In similar fashion, Locke supposed that the simple idea of unity, repeatedly recombined with itself, provides the entire content for the simple modes of number, including even that of infinity. [Essay II xvi - xvii] In every such case, the complexity of the simple mode arises only from an iteration of a single simple idea, upon whose content the complex idea therefore relies.

Mixed modes, on the other hand, are complex ideas whose components include several distinct simple ideas, often including those derived from different experiential sources. Although such ideas can be acquired through observation of their instances in our experience, Locke supposed that they are much more commonly manufactured by the mind as complex ideas before we first apply them to the world. [Essay II xxii 1-2] This is an important feature of our complex ideas of mixed modes. Among them are included the conceptions we form of human activities, which typically involve not only ideas of sensation describing the overt action but also ideas of reflection regarding the intention with which it is undertaken. For Action being the great business of Mankind, and the whole matter about which all Laws are conversant, it is no wonder, that the several Modes of Thinking and Motion, should be taken notice of, the Ideas of them observed, and laid up in the memory, and have Names assigned to them; without which, Laws could be but ill made, or Vice and Disorder repressed. Nor could any Communication be well had amongst Men, without such complex Ideas, with Names to them: and therefore Men have setled Names, and supposed setled Ideas in their Minds, of modes of Actions distinguished by their Causes, Means, Objects, Ends, Instruments, Time, Place, and other circumstances; and also of their Powers fitted for those Actions. [Essay II xxii 10] The idea of stabbing, for example, typically involves a deliberate act of penetrating flesh without any presumption about the instrument employed to do so. In such cases, it may be worthwhile for legislative bodies to conceive of such actions and condemn them as criminal without having to have experiential evidence of their first occurrence. [Essay III v 2-6] The only thing that matters for the formation of a mixed mode, on Locke's view, is the convenience of its use for us. This clearly varies among distinct cultures, since the frequency with which we have occasion to notice, denominate, and evaluate actions of particular sorts often depends upon their social context. Invented wholly for our own use in matters of moral and legal import, the ideas of mixed modes have only that specific content we choose to give them. [Essay II xxii 5-12] As we'll see next time, the freedom with which such ideas are manufactured renders problematic our ability to use a common vocabulary in reference to them.

Complex ideas of relations derive from the mental operation of comparing distinct ideas without thereby combining them together into an entirely new whole. [Essay II xii 1, 7] Since this comparison considers each of the ideas in light of its extrinsic conformity with the other, it is commonly expressed by means of one of a pair of reciprocal relative terms—"parent"/"child" or "cause"/"effect," for example. This is not invariably the case, however, so we must be alert to the possibility that apparently positive terms implicitly signify comparison with something else: "old" or "imperfect," for example. The tip-off to such cases, on Locke's view, is that (as in the case of relations of the other sort) removal of the correlative destroys the relation completely, rendering the term inapplicable. [Essay II xxv]

Many of our ideas of relations are acquired through sensory experience of the modes of time, place, and number. The correlative ideas of cause and effect are a special instance, on Locke's view: observing the changes produced in one thing by the operation of another, we form a sensitive notion of the causal relation even though we have no conception of the underlying mechanism at all. [Essay II xxvi 1-6] Because language is devised for our convenience in satisfying the needs of ordinary life, things that are commonly connected in observation give rise to our complex ideas of natural relations. But we also conceive of instituted or voluntary relations between things that are corrleated with each other only by virtue of our own personal or social agreements. Among these, on Locke's view, the most important and vital are the moral relations drawn between the complex ideas of specific human actions, together with their circumstances and goals, and the moral rules by reference to which we evaluate them. [Essay II xxviii 1-4] We'll look at these much more closely later on.

According to Locke, the complex idea of a substance is a collection of simple ideas that is believed capable of existing independently. Observing in experience that several features recur together frequently, we suppose that there must be some common subject that has all of them. Thus, for Locke, the "Notion of pure Substance in general" is nothing but the assumption of an unknown support for a group of qualities that produce simple ideas in us; our only notion of this putative "substratum" in itself is the confused notion of "that which" has these features, or "something, I know not what." [Essay II xxiii 1-3] Locke consistently ridiculed the use of such scholastic terms as "Inhaerentia" and "Substantia" as pointlessly circular efforts to provide a positive idea of something for which, in fact, "we have no Idea of what it is, but only a confused obscure one of what it does." From this, he supposed, we can infer little of the metaphysical nature of reality. [Essay II xxiii 17-20] Nevertheless, just as we combine simple ideas of qualities and powers to form the complex ideas of individual substances, we also gather many distinct things under the even more complex ideas of collective substances—army, world, and universe. [Essay II xxiv 1-3]

To the confused idea of substance in general, Locke held, we add either the ideas of regularly observed sets of sensory qualities, in order to form the notion of a bodily substance, or the reflective ideas of various kinds of mental operation, in order to form the notion of a spirit or thinking thing. Since the underlying nature of the substrate is equally obscure in both cases, the "secret and abstract Nature of Substance in general" is equally unknown to us for substances of either sort. [Essay II xxiii 4-6] We do have clear and distinct ideas of the primary qualities of both bodies (solidity and impulse) and mind (thinking and motivity) even though our sensation and reflection offer no insight into "the internal Constitution, and true Nature of things," so that the relation and/or independence of bodies and minds remains an open question. [Essay II xxiii 29-32] Thus Locke argued that even though we are often more familiar with material objects than with spirits, our ideas of them may be no clearer, and the customary prejudice in favor of bodily substance is philosophically indefensible. [Essay II xxiii 15-16]

With respect to bodily substances, the most accurate complex idea we could form, on Locke's view, would be one that includes the ideas of those active powers and passive capacities that it exhibits in its interaction with other things—the magnetism of the lodestone and the flammability of wood—along with its characteristic weight, color, or heat. Since he believed the primary qualities of bodies to reside in the unobservable texture of their sub-microscopic parts, Locke found it natural to suppose that we classify distinct kinds of body by reference to the secondary and tertiary qualities these unknown features produce in us or other things. [Essay II xxiii 7-10] To express one of Locke's favorite examples in our own idiom, we were able to identify genuine pieces of gold for centuries before we had any clue to its atomic structure.

Several late chapters of Book II are devoted to a detailed discussion of the success with which ideas of various kinds perform their representative functions. The point here is that since we use ideas as signs, it is vital to be aware of the likelihood that they do actually point beyond themselves to their intended referential objects. The extent to which they serve these functions will determine the reliability of any knowledge we try to acquire about those objects.

Locke explained the clarity of ideas by analogy to visual perception: just as an object is seen clearly when viewed in suitable light, a clear idea is one of which we have a "full and evident perception," whose content is present before the mind. An idea is distinct, on the other hand, when it is perceived to differ from all others. [Essay II xxix 2-5] Locke had no stake in differentiating sharply between the clarity and distinctness of ideas. In the Essay's fourth edition, he made a half-hearted effort to substitute the single adjective "determined" in place of the customary pair "clear and distinct." His central concern was with the failure of this representational function, in ideas that are confused. This occurs most frequently with respect to complex ideas, whose simple components may be too few or too poorly organized to determine their content precisely. The problem, Locke argued, is that we often use words as if we knew their significance when, in fact, the ideas associated with them are not fully conceived. [Essay II xxix 7-11] We'll consider this issue more fully next time, but it's worth noting that Locke believed that many apparently intractable philosophical disputes arise from a failure to employ words to signify clear and distinct ideas. His examples here all arise from the wide-spread failure to hold in mind a correctly-formed, determined idea in association with the word, "infinity." [Essay II xxix 13-16]

In Locke's taxonomy, and idea is said to be real (as opposed to fantastical) so long as there is something that it represents. Notice that the accuracy of representation is not at issue here at all. Even if the idea fails to correspond to its object, it is real provided only that the object does exist. Since simple ideas are passively received in the mind, for example, they must be caused by something real, even though—as we've already seen—in the case of secondary qualities they fail to resemble their causes. [Essay II xxx 1-2] Because our complex ideas of modes and relations are not supposed to refer to anything else beyond their own archetypal content, they are all real as well. Ideas of substances, however, are intended to refer to existing things and are therefore fantastical if things with the appropriate combinations of features do not in fact exist. [Essay II xxx 4-5]

An idea is further said to be adequate only if it represents its intended object fully and perfectly; inadequate ideas convey the nature of their objects only partially. [Essay II xxxi 1] Locke insisted that all simple ideas are adequate, though doing so required some fancy footwork with respect to our ideas of secondary qualities. Properly understood, he argued, simple ideas represent whatever power it is that produces them in us, whether or not the idea resembles that power—which, in the case of secondary qualities, it does not. As with the reality of ideas, so with their adequacy the vital point for Locke was the causal process by means of which we acquire them; our lack of voluntary control over that process forestalls any possibility of mistake or erroneous judgment. [Essay II xxxi 2, 12] Because complex ideas of modes and relations are assembled by the mind without reference to any external archetype, they are their own archetypes, which they cannot fail to capture adequately. Although our communication with each other about such ideas (especially in the case of mixed modes) may falter because we do not agree about the signification of words we both employ, the ideas themselves are invariably adequate. [Essay II xxxi 3-5, 14] Once again, it is complex ideas of substances that are unreliable; such ideas, according to Locke, have a double intended reference but are inadequate in both respects. If they are supposed to represent the substantial forms of existing things, our ideas of substances are inadequate because (on the corpuscularian theory) these real essences are unknowable. Even when considered more modestly, as collections of properties that co-exist in a common substrate, our complex ideas of substances are merely partial approximations, since it is clear that they include only those features we have most commonly observed; it always remains possible for us to be surprised by the discovery of another property that belongs just as surely in the same being. [Essay II xxxi 6-11] As always, ideas derived from experience can be no more adequate than the experience itself, and with respect to natural things our experience is limited.

Although he maintained that truth and falsity are most properly regarded as characteristic of propositions, Locke granted that ideas are sometimes said to be true or false (better: right or wrong) by virtue of the role they play in the formation or assertion of such propositions. [Essay II xxxii 1-5] It is the relation between ideas and the words used to signify them that matters for this representational criterion. Because of their familiarity in our experience and the frequency of our discourse about them, our ideas of natural substances and their qualities are often truly signified by the common terms we employ to designate them. [Essay II xxxii 6-10] It is the ideas of mixed modes, manufactured independently in the minds of individual thinkers and lacking any external referent, that are most commonly false in this linguistic sense, because we do not agree on the signification of their names. [Essay II xxxii 11-13]

On the whole, then, Locke believed that ideas provide a vital but imperfect foundation for human discourse and knowledge. As we continue our study with an examination of his philosophy of language and of knowledge, we'll be reminded of the limitations imposed by the reliability of ideas of each type.

Having explained the origin of our ideas and the use of words to signify them, Locke was prepared to consider the nature of human knowledge. He began with a simple definition: Knowledge then seems to me to be nothing but the perception of the connexion and agreement, or disagreement and repugnancy of any of our Ideas. In this alone it consists. [Essay IV i 2] This definition gives rise to an obvious objection: if all human knowledge were wholly concerned with ideas, then it would lack an adequate foundation in reality and there would be no difference between the wise contemplation of philosophers and the fanciful but coherent imagination of the insane. Locke's response was two-fold: with respect to general knowledge, he boldly defended its detachment from anything other than our ideas, but with respect to knowledge of particulars he tried to show that there is a legitimate cognitive state that "goes a little farther than bare Imagination." [Essay IV iv 1-2]

Actual knowledge occurs only when we are actually perceiving the agreement in question, Locke supposed; our inclination to assent to what we've known in the past but are not presently attending to he called habitual knowledge. This capacity is clearly important for the development of learning generally; we can't focus on everything at once, and it makes sense to rely upon our memories for general truths we have mastered on some occasion in the past. [Essay IV i 8-9] Thus, it makes sense to say that I know (habitually) the multiplication tables even at moments when I'm thinking about other things, provided that I can, upon challenge, call to mind the product of 8 and 7. The more crucial issue about human knowledge is to explain how it occurs in the first place.

One account Locke unambiguously rejected from the outset is the supposition that human knowledge is innately inscribed. Noting the remarkably wide-spread agreement of individual human beings in their acceptance of both speculative and practical principles, the innatist argues that universal consent implies an innate origin. Locke's response was two-fold: He denied the supposed fact of universal consent, supposing this to demonstrate the falsity of the innatist view. What is more, Locke argued that if there were any genuine instances of universal consent, they would more naturally be explained by universal possession of an intellectual faculty or by acquisition through some universal experience. [Essay I ii 1-2]

Granting that if general truths about logic were innately know by all human beings, then they must also be universally accepted, Locke emphatically denied the consequent. If the innatists were correct, then children (and mental defectives) would be the most pure and reliable guides to logical truth; but they are not. [Essay I ii 24-27] Of course the innatist reply to such counter-examples is to suppose that assent to innately inscribed principles is delayed until each individual is able to employ the faculty of reasoning. But why should this be? Either reason is necessary for the discovery of such principles, in which case they are not innately known, Locke argued, or else reason and logic are merely coincidental features of human development, in which case both seem frivolous. [Essay I ii 6-13] Surely, in fact, the use of reason is properly concerned with our assent to general truths.

Locke agreed with the innatists that there is a significant distinction between truths to which we assent immediately upon first framing them on the one hand and, on the other hand, truths that require our careful consideration. But this distinction between self-evident and acquired knowledge, he supposed, doesn't correspond to the innatist identification of the most fundamental truths. We assent to specific instances at an earlier stage of development than to speculative maxims themselves, and this is most easily explained by the fact that we acquire the particular ideas involved in the former long before we have manufactured the abstract ideas required for the latter. [Essay I ii 14-19] Careful attention to the development of knowledge in individual cases clearly shows it to involve the gradual acquisition of the requisite ideas, perception of whose agreement or disagreement constitutes knowing in each instance.

Locke's definition of knowledge as perception of the agreement (or disagreement) of ideas clearly indicates two fundamental criteria for acquiring knowledge: first, we have to have the requisite ideas, then we also have to perceive the connection between them. Failure on either of these counts will leave us short of the certainty characteristic of genuine knowlege. [Essay IV iii 1-2] The extended genetic account of our ideas in Book II of the Essay was designed to assist in satisfaction of the first criterion, by helping us to acquire a suitable stock of clear and distinct ideas about many things. But even where our ideas are amply clear, we may fail to achieve certain knowledge if we are unable to recognize the ways in which they are related to each other. [Essay IV ii 15]

Consider, for example, how the two criteria apply to specific areas of possible human awareness: Our passive reception of simple ideas from external objects provides only practical assurance of the existence of the objects themselves, so our knowledge of material qualities and substances is always limited in certainty. But non-substantial complex ideas refer to nothing outside themselves, so any connections we perceive among them are "infallibly certain." For this reason, as we'll see later, Locke held that all mathematical and moral reasoning is characterized by perfect certainty. [Essay IV iv 4-8]

What Locke's definition clearly demands is that all knowledge is relational in structure, or propositional in form; it invariably involves "the joining or separating of Signs, as the Things signified by them, do agree or disagree with one another." [Essay IV v 2] But since both ideas and words are properly understood as signs, Locke believed it vital in principle, though often difficult in practice, to distinguish between truth in thought and truth in language. Ultimately he distinguished between mental propositions in which ideas are perceived to agree and verbal propositions that affirm the agreement of words. In both cases, the criterion of truth is that the relation of the signs conforms to a more fundamental agreement that holds among the things they signify. [Essay IV v 3-5] Naturally the two kinds of proposition are connected with each other. Certainty about the truth of a verbal proposition requires that it accurately express the agreement of the ideas signified by its terms; certainty about the knowledge itself further requires that we actually perceive that agreement among ideas. [Essay IV vi 3]

Locke distinguished four types of agreement or disagreement that may be perceived in human knowledge: Knowledge of identity and diversity rests solely upon our recognition of the distinctness of each idea from every other; knowledge of relation employs positive, non-identical connections among ideas; knowledge of co-existence perceives the coincident appearance of a collection of qualities; and knowledge of real existence presumes some connection between an idea and the real thing it represents. [Essay IV i 1] He further supposed that these types of knowledge can occur in any of three degrees:

Intuitive knowledge is the irresistable and indubitable perception of the agreement of any two ideas without the mediation of any other. This is the clearest and most perfectly certain of all degrees of human knowledge. It accounts for our assent to self-evident truths and serves as the foundation up-on which all other genuine knowledge must be established. [Essay IV ii 1] Intuition is most common in our knowledge of identity and relation among clear ideas, but (following Descartes) Locke also supposed that each thinking being has an intuitive knowledge of its own existence. [Essay IV ix 3]

Even when the agreement of ideas is not intuitively obvious, it may be possible to discover a series of intermediate ideas by means reason establishes a connection between them. The resulting demonstrative knowledge of the agreement of the original ideas shares in the certainty of the intuitive steps by means of which it has been proven, yet there is some loss of assurance resulting from the length of the chain itself. [Essay IV ii 2-7] The success of the entire process depends upon our having clear ideas at each step of the process of demonstration and upon our ability to perceive the agreements between them, and both of these conditions are specific to the nature of human intellectual abilities. [Essay IV iii 4, 26-28]

The most common area of demonstrative human knowledge is mathematics, where our possession of distinct ideas of particular quantities yields the requisite clarity, disciplined reasoning helps to uncover the intermediate links that establish knowledge of identity and relation, and a perspicuous system of symbolic representation helps us to preserve the results we have obtained. [Essay IV ii 9-10] But Locke supposed that we may be capable of demonstrative knowledge of moral relations as well, provided that we take care in the formation of abstract ideas of the mixed modes of human action. Our only demonstrative knowledge of real existence, he supposed, is that we can have with respect to God. [Essay IV x]

Although only intuition and demonstration offer certain knowledge of general truths, Locke supposed that sensitive knowledge provides some evidence of the existence of particular objects outside ourselves. Although it is not always true that there must exist an external object corresponding to each of our ideas of sensation, Locke argued that veracious cases are different enough from illusory cases to warrant an inference to the real existence of their objects, especially when the accompanying perception of pleasure or pain serves as a reliable guide to the practical conduct of human life. [Essay IV ii 14] Locke had serious reservations about the reliability of our sensitive knowledge of the natural world, and we'll return later to the chapter of the Essay he devoted entirely to an analysis of its difficulties. [Essay IV xi]

One of the most basic themes of Locke's epistemology is that since we cannot know everything, we would be well-advised to observe and respect the extent and limitations of human knowledge. Given the basic definition of knowledge as perception of the agreement of our ideas, it follows that we fall short of knowing whenever we lack ideas or fail to perceive their agreement. Thus, intuition extendes only to the identiy and diversity of ideas we already have; demonstration extends only to ideas between which we are able to discover intermediaries; and sensitive knowledge informs us only of the present existence of causes for our sensory ideas. [Essay IV iii 3-5] Awareness of our limitations, Locke proposed, should forestall haste, laziness, and despair in our natural search for the truth about the most vital issues into which human knowers can fruitfully inquire. [Conduct 39-43]

Applying the human faculty of reason to the pursuit of knowledge, properly defined, reveals the limitiations within which we must work: We cannot achieve knowledge of things—such as infinity or substantial real essences—for which we lack clear, positive ideas. Indeed, having ideas will not be enough to secure knowledge if—as in the case of human actions—they are obscure, confused, or imperfect. Given faulty memories, we may also fail to achieve knowledge because we are incapable of tracing long chains of reasoning through which two ideas might be demonstrably linked. In a more practical vein, rational knowledge cannot be established upon false principles—such as those borrowed from conventional wisdom. Finally—in the effort to achieve philosophical or scientific certainty—our efforts to employ reason are commonly undermined by the misuse or abuse of language. [Essay IV xvii 9-13]

Most particularly, on Locke's view, it is difficult to secure the reality of human knowledge in any evidence of its conformity with the nature of things themselves. We readily assume that passively-received simple ideas must be providentially connected with their objects, and since complex abstract ideas are of our own manufacture, it is our own responsibility to ensure their reality by a consistent use of the names by which we signify their archetypes. But complex ideas of natural substances are intended to represent the way existing things are independently of our perception of them, and of this the content of our ideas never provides adequate evidence. [Essay IV iv] These difficulties trouble all four types of knowledge.

Since knowledge of identity and diversity involves only a recognition that particular ideas are distinct, it is immediately evident whenever we have clear and distinct ideas; such knowledge must be among the earliest that any of us ever achieve. [Essay IV vii 9-11] Even otherwise ignorant beings might well be capable of perceiving the disagreement of ideas at this level, Locke pointed out, so

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