 Further reflection, however, shows us that a soul (and especially a human soul) is a complicated thing. It has a variety of capacities, which seem to be really distinct from one another and possibly from the soul itself. Some of these capacities even seem to be contrary to one another. For example, a soul seems to be capable of psychic conflict. There is a lot of pressure then put on this putatively simple, yet obviously complicated thing, and therefore it is hardly surprising that there is a robust medieval debate over the nature of the parts or powers of the soul (see Perler 2015). Some think that the pressure is too great to sustain and that one substantially simple thing cannot be the subject of all these capacities. Two common reasons are given for the soul’s being really divided in a substantial manner. First, it seems that really distinct operations must stem from really distinct sources, and since operations are accidents of substances, this would seem to imply that the operations of a soul must stem from distinct substantial sources. Second, according to Aristotle, the rational capacities do not require a bodily instrument to operate, where the other capacities do. This leads some medieval Aristotelians to conclude that whereas the non-rational capacities arise from the matter of the thing, the rational capacities must come from some external source. This, in turn, suggests that a human being has at least one substantial form that is rooted in and arises from the matter, and another substantial form that comes from an external source. If all of this is right, then humans at least either have several souls, or one substantially composite soul, where the parts are viewed as “incomplete” substances that combine to form one “perfected” soul (see Pasnau 1997; Perler 2014). Others, and most famously Aquinas and his followers, hold that a living thing can only have one substantial form and therefore only one soul (Pasnau 2002, 126–30). These “unitarians” must come up with an answer to how the powers do not in fact divide the soul into souls, especially those unitarians like Aquinas who hold that the powers of the soul are really distinct from the soul itself. (In fact, according to King (2008) the dominant view in the medieval period is that the soul’s powers are really distinct from the soul’s substance.) A commonly proposed solution is that the powers “flow out” from the soul’s essence and that the subordinate powers of the soul are “virtually present” in the soul (see Shields 2014 for an analysis of Suárez’s version of this solution; also Perler 2015). 