The view on their argument here based on Istihsan

Sometimes the masses argue for the washing of the feet, they see it as most suitable for the feet rather than wiping [the feet], just as wiping is more suitable for the head rather than the washing because mostly, the filth on the feet cannot be cleaned except by washing them as opposed to the head, it can be cleaned mainly by wiping.

They said that there is nothing to prevent the benefits, as understood by the intellect, to be causes for the prescribed worship. Thus the law looks at [an act] in two meanings: the general benefit and the benefits derived from that [act of] worship. By the general benefit they mean the benefits that can be sensed [by the intellect] and by [the benefits] of worship they mean what refers to the purification of the soul.

I say: We believe that the lawgiver was lenient to His slaves in everything that He prohibited them from, unless where there was corruption for them. Despite that, He did not impose a single shari'a ruling on them nor did He command them to do anything unless it was to their benefit. He did not make these rulings dependent on the slaves' views of the benefits and corruption, rather, He imposed the rulings on them with strong proofs which He prescribed for them. He (Allah) has not given any alternative to them nor has He prescribed anything equivalent. The first source of those rulings is the book of God, the most Mighty and Glorious, in which He has commanded the wiping of the head and the feet in the wudu, so it is essential to abide by His ruling. As for the cleanliness of the foot from filth, it is necessary to guard against it before wiping on it acting in accordance to special proofs, which show that it is a prerequisite that the parts where wudu is done must be pure before starting it.

Perhaps the washing of his feet by the Prophet of God (P), as reported in the traditions, was due to this reason. Maybe he did it to cool his feet or because he was intense in observing cleanliness after completing the wudu and God knows best.

Notice

Ibn Maja has reported concerning the washing of the feet in his Sunan from Abu Ishaq on the authority of Abu Hayya, he said: "I saw 'Ali doing the wudu and he washed his feet to the anklebones and then he said: 'I wanted to show you the purification of your Prophet (P)'" When he completed this citing tradition, al-Sanadi said in his comments on the Sunan: "Since the washing has been narrated from 'Ali, this is a major refutation against the Shi'a who believe in the wiping of the feet." He further said: "Therefore the author has mentioned it on the authority of 'Ali, and has started the chapter with it. The author has done well and he excelled in reporting the hadith of 'Ali in this chapter, may God reward him for that." He said: "The apparent meaning from the Qur'an necessitates the wiping as has been reported from Ibn 'Abbas but it is obligatory to interpret it as referring to washing." These are his words, may God forgive him, Imam Ibn Maja and all the scholars of the masses. They know that this tradition is invalid because its chain of transmission is invalid due to several reasons.

Firstly, Abu Hayya, the reporter of this tradition, is completely unknown. Al-Dhahabi mentioned him in the section of the patronymics (al-kuna) in his Mizan and has stated that he is not known. Then he cited Ibn Madini and Abu Walid al-Fardi saying that he (Abu Hayya) was an unknown person. Then he said: "Abu Zar'a said he is not mentioned." I say I have investigated Abu Hayya extensively and I have not found a discussion except that it mentioned him as an unknown person; perhaps some fapicators of traditions have fapicated him, and God, the Almighty, knows best.

Secondly, this tradition is only reported by Abu Ishaq. He grew old and he used to forget and confuse traditions so people abandoned him. Nobody except Abu al-Ahwas and Zuhayr b. Mu'awiya al-Ju'fi narrated from him, and so people found fault with him because of that. No wonder, if a traditionalist mixes up [traditions], all his traditions which are not known to have been transmitted before his becoming confused become invalid; [this is applicable] whether it is known that it has been reported after his confusion (like this tradition) or the date of the tradition being reported is not known. [This is] because general knowledge in doubtful specific circumstances necessitates keeping away from all the peripheral matters as has been established in usul al-fiqh (the science of deriving juridical principles).

Thirdly, this tradition contradicts established traditions from the Commander of the Faithful and from his sons, the ahl al-bayt of the Prophecy and the place where Prophecy was revealed and the frequenting place of the angels and where revelation was revealed; it also contradicts the book of God, the Almighty and Majestic; so let us discard it.

Adapted from: "Questions on Jurisprudence" by: "Abdul Hussein Shareefaldin Al-Musawi"