Wisdom of wudu…
“As for His obligating the washing of limbs from which gas is not passed, and not obligating the washing of the organ from which it is passed (during wudu’) – then, how much is this in accordance with wisdom, and how compliant is it with the fitrah! So, if it is asked: why is wudu’ performed for these limbs, which are apparent, and not for the hidden buttocks, considering that the buttocks are more deserving of the wudu’ than the face, hands, and feet? This is a backwards question from a backwards heart, as from the virtues of the Shari’ah is that wudu’ is for the outer, apparent limbs, and that the one most deserving of it is the leader of these limbs in contemplating and acting, and that is the face, whose cleanliness and brightness is a sign of the cleanliness of the heart. After it come the hands, and they are the instruments of of grasping, giving, and taking. Therefore, they are the most deserving organs after the face of being clean and pure. And since the crown of the head is the focal point of emotion, and the most elevated and honored of organs, it is most deserving of being clean. However, if washing it was legislated during wudu’, this would be a cause of great hardship and trial. So, it was legislated to simply wipe over it, and this was a light and merciful substitute for washing it, just as wiping over footwear took the place of washing the feet.
Someone might ask: and how does wiping over the head and feet bring about any washing or cleanliness?
Such a questioner does not realize that touching a limb with water – in fulfillment of the Command of Allah, obedience to Him, and worship of Him – affects it’s cleanliness and purity in a way that would not occur by washing it with water and soap without this intention. This is all referred back to one’s sound taste and straight nature, just as rubbing your face with dirt (during tayammum) – in fulfillment of the Command of Allah, obedience to Him, and worship of Him – increases it in purity, cleanliness, and gracefulness that is obvious to any onlooker.And since the feet are what usually touch the ground, and are in contact with filth that the rest of the limbs do not come into contact with, they are most deserving of being washed, and the one who limits himself to wiping over them without a valid excuse has not been granted understand of Allah and His Messenger.
So, these are the logical and material reasons for specifying these limbs in exclusion to all other limbs for wudu’. As for the spiritual perspective, these limbs are the instruments by which the servant carries out what he wants to do, and by them, Allah – the Exalted – is disobeyed and obeyed. So, the hand is used to grab, and the foot is used to walk, and the eye is used to look, and the ear is used to listen, and the tongue is used to speak. So, in washing these limbs – in fulfillment of the Command of Allah, and establishing the worship of Him – we are removing the muck and filth of disobedience and sin. And the Messenger of Allah indicated what is close to this meaning, as in the authentic hadith reported by Muslim in his ‘Sahih’ (# 832), on the authority of ‘Amr bin ‘Abasah, that he said: “O Messenger of Allah, tell me about wudu’!” So, he said: “There is none amongst you who begins the wudu’, and washes his mouth and nose, except that the sins committed by his face, mouth, and nostrils are washed out. Then, when he washes his face as Commanded by Allah, the sins of his face are washed out with the water from the sides of his beard. Then, when he washes his hands up to elbows, the sins of his hands are washed out through his fingers with the water. Then, when he passes his wet hands over his head, the sins of the head are washed out through the ends of his hair with the water. Then, when he washes his feet up to the ankles, the sins of his feet are washed out through his toes with the water. Then, if he stands up for prayer and praises Allah, glorifies Him, proclaims His Greatness as He deserves, and devotes his heart wholly to Allah, he emerges free from sin, just as the day he was born.” …and what can be derived from this is that the purpose of rinsing the mouth is just like the purpose of washing the face and hands, and that the need of the tongue and lips to be washed is just like that of the remaining organs and limbs. So, there is none with a more backwards heart, corrupted fitrah, and invalid logic than the one who says that the buttocks are more deserving of being washed than these limbs…? This is in addition to the fact that washing these limbs with the intention of worshipping Allah leads to the widening and strengthening of the heart, joy of the soul, and energy of the limbs, and this is why these limbs have been designated out of all the other limbs, in that they are to be washed in exclusion to the others, and Allah is the source of success.”
['I'lam al-Muwaqqi'in'; 2/58-60]