As was pointed out, once the complete cause is realized, the effect will also be realized; and whenever we come across an effect, we reach the necessary conclusion that there must have been a cause previously realized. Therefore, the relation of necessity between cause and effect is a real one; that is, it is a kind of relation which cannot possibly be changed into some other kind of relation. In other words, the relation between cause and effect is like the relation between natural integers.
Just as it is necessary for number 4, for example, to occur between the numbers 3 and 5, or otherwise the series is rationally distorted, so also the necessary relation between every cause and effect is that of succession in which the position of any element in the series cannot be changed. According to what has been said, any doubt about the existence of necessary relation between cause and effect is, in fact, a doubt about the validity of the principle of causality itself.
Just as it is necessary for number 4, for example, to occur between the numbers 3 and 5, or otherwise the series is rationally distorted, so also the necessary relation between every cause and effect is that of succession in which the position of any element in the series cannot be changed. According to what has been said, any doubt about the existence of necessary relation between cause and effect is, in fact, a doubt about the validity of the principle of causality itself.