 Many kinds of item can be wholes, and many kinds of item can be parts of these wholes. In general, any item that composes a whole is a part, and any item that is a product of a division of some whole is a part of that whole. The only clear restriction on what can be a part is that no part is identical to its whole. In other words, no medieval philosopher countenances what contemporary mereologists call improper parts. (On the contemporary notions of part, or improper part, and proper part consult the entry on contemporary  mereology,  and also Simons 1987, 9–11.)