 These remarks on the relations between forms and substances bring us to the core of Alyngton’s ontology: the doctrine of substance, developed in the fifth chapter of his commentary on the Categories. Alyngton’s treatment can be divided into two main parts. (i) The first attempts to clarify what characterizes substance, and therefore what falls by itself into that category; (ii) the second is concerned with the distinction between primary (or individual) and secondary (or universal) substances.