 There are other good reasons why medieval philosophers might be motivated to distinguish the integral parts of a thing from the substantial parts of the same thing. Firstly, the form does not behave like the integral parts of the thing, since for many Aristotelians (and especially, as we saw above, for Aquinas and other unitarians) the form has the unique capacity to be present “as a whole” in each material part of the thing. Integral parts, by contrast, are thought to be unable to be present in more than one place at the same time, and for this reason, the integral parts of a thing are said to sit side-by-side, or “part outside of part” (pars extra partem). Secondly, substantial parts require something else to perfect them (see Albert of Saxony Sophismata 49). Matter by itself is incomplete; it is only potentially this or that thing. Matter needs a form in order to actually be this or that thing. Form, too, is in a sense an incomplete substance. A dog’s form, for instance, is in need of some matter in order to be the form of this dog. But more fundamentally, given the Aristotelian critique of Platonic forms–specifically, the critique of the notion that a form can exist independently of matter–many medieval thinkers will insist that a form is incomplete in the sense that it cannot exist unless it is part of a substantial composite. Finally, distinguishing between substantial parts and integral parts captures an important intuition about substances: Socrates can lose and gain matter without compromising his existence. But if Socrates were to lose either his matter in total or his substantial form, he would cease to exist. 