 He goes on to associate potentiality with matter, and actuality with form, and draws the distinctions we would expect between the various kinds of matters and forms. He also remarks at this point that being is of two kinds, and that the being of a substance is “something different” from accidental being. This signals his commitment to Aristotle’s doctrine of the multivocity (or homonymy) of being. What it means for a substance to exist is different from what it means for an accidental aggregation of a substance with its accidents (say, a pale man) to exist. In general, what properly exists is substances, whereas “forms, accidents, and other things of this sort are called beings not because they themselves exist, but because it is by them that something exists” (ST 1a 45.4c). Moreover, even among substances, God does not exist in the same way that creatures do, but only in an analogous way (§2). This denial of the univocity of being was contentious among later medieval authors (most notably  John Duns Scotus)  and its correct interpretation remains controversial today (Brower 2014; Pasnau 2018). Even more problematic is Aquinas’s conception of prime matter, which he characterizes as pure potentiality: “being in actuality is incompatible with matter’s nature, since matter is a potential existent by its very nature” (Quodlibet III.1.1). Although later medieval Aristotelians accepted that substantial change requires a material substratum, hardly anyone endorsed Aquinas’s suggestion that it is pure potentiality, because they were convinced that what lacks actuality does not exist. Readers of Aquinas have disputed how he himself could have evaded that outcome (Brower 2014 ch. 5), or whether perhaps it is an outcome he intended (Pasnau 2002 ch. 1).