Alean hylomorphism stands as a metaphysical holy grail. An embarrassing feature of traditional hylomorphic ontologies is prime matter. Prime matter is so basic that it cannot be examined (in principle), and its engagement with the other hylomorphic elements is far from clear. One particular problem posed by prime matter is how it is to be understood both as a principle of individuation for material substances and as pure potency. The author presents Thomas Aquinas's way of squeezing some intelligibility out of prime matter by modeling it on the idea of logical genus. Such a modeling provides insight into understanding prime matter as substratum, as maximally indeterminate, and as ontologically vague. One of the unusual but exciting things that fall out from this analysis of prime matter is the entirety thesis: "For any substance x, if x has prime matter then the prime matter of x is the same* as x," where 'same*' is understood as "indeterminately identical."