“This perceived modularity of the human body is what enables, I suggest, one of the most perplexing rabbinic (or protorabbinic) innovations, namely, the ruling that one’s hands are constantly impure (in a low degree) regardless of the impurity status of the person as a whole. One’s entire body can be certifiably pure, but unless one has just washed one’s hands this very instant, his hands are considered to be “second to impurity” in such a way that they disqualify a heave-offering just by touching it, and if one’s hands are wet, they also transmit impurity to ordinary food. The reason for this constant status of impurity, as stated in the Mishnah, is that the hands always ‘busy themselves’ (she-ha-yadayim ‘asqaniyot), that is, one’s hands are likely to do things and touch things of which their “owner” is not aware. In other words, the rabbis assume a certain dissociation of the hands from the rest of the body insofar as the hands have ‘a will of their own,’ and therefore ascribe to the hands an impurity status that is independent of the rest of the body.”

–Mira Balberg, Purity, Body, and Self in Early Rabbinic Literature pp.59-60