Derekh Hashem, Part Four, On the Daily Order of Prayer

Washing of the hands (netilat yadayim): Behold the first act is the purification of the hands, because they are that which became impure and upon which evil spirit rested; so one must drive it away from them and purify them. And behold the Creator, may He be blessed, embedded that they be driven away through the appropriate washing, as they, may their memory be blessed, taught us. And it comes out that his whole body is purified by this, just as all of it had been impure from the resting of evil spirit on [the hands]. And there is also a refinement of the whole entire creation with this matter, to be purified from the impurity of the night and to exit from its darkness. And behold they also connected a man's cleaning his body after relieving himself to this [action]. And it comes out that he is [then] totally purified and prepared to come close in front of his Creator.