 Many of the heterogeneous parts of Socrates are best defined in terms of their function, not their measure. For example, hands are discriminated from feet based on what functions they perform for an animal. Many medieval philosophers believe that these functions are provided either by the form or the soul of the animal. For this reason, many medieval philosophers call functionally defined parts “formal parts”, or parts secundum formam, but in order to avoid any confusion, I will refer to parts of this sort as functional parts. The fourteenth-century philosopher Walter Burley tells us that the functional parts “remain the same so long as the whole remains the same and complete” (De toto et parte, 301). In other words, so long as Socrates’ soul occupies his body, and provided the hand is not cut off, Socrates’ hand remains a hand. Burley contrasts functional parts with material parts, and he places homogeneous parts such as flesh, bone, and blood under this category. Socrates’ material parts are in constant flux; Socrates is constantly losing and replacing bits of flesh and blood.