 Since Burley conceives of primary substances as the ultimate substrates of existence and subjects of predication in relation to anything else (EP, ch. de substantia, fols. 24va–b, and 25va), the only way to demonstrate the reality of beings in other categories is to treat them as forms and attributes of substances. Since Burley wanted to preserve the reality of quantity and its real distinction from substances and other accidents, he insists that quantity is a form inherent in the material part of a composite substance (EP, ch. de quantitate, fol. 29rb). This is problematic, however, for if the highest genus of the category is a form, Aristotle’s seven species of quantity (line, surface, solid, time, space, number, and speech), mentioned in the sixth chapter of the Categories, are not. Burley tries to meet this difficulty by reformulating the notion of a quantified thing (quantum). Encouraged by the Aristotelian distinction between strict and derivative quantities (Categories 6, 5a38–b10), he distinguishes two different modes of being quantified: in-itself (per se) and in-virtue-of-something-else (per accidens). The seven species of quantity are quantified by themselves, whereas other quantified things (e.g., corporeal substances) are quantified in virtue of one or more of the seven species (EP, ch. de quantitate, fol. 29va). In other words, Burley considers the seven species not as quantitative forms, but as the most proper and primary bearers (supposita) of the quantitative properties revealed by the supreme genus of the category. Any other kind of quantified thing is simply a subject (subiectum) of quantitative forms.