Summa of Logic

Ockham's comprehensive textbook of logic, in three parts: on terms, on propositions, and on arguments, the last covering syllogisms, consequences, demonstration and fallacies. The first part contains his philosophy of language and much of his metaphysics. Spoken and written words signify by convention, but they are subordinated to concepts in the mind, which make up a natural mental language. Universals are not things existing outside the mind but signs: concepts that can be predicated of many individuals. Through the theory of supposition, which explains what a term stands for in a given proposition, Ockham shows how to read statements about universals, categories and abstract nouns without positing extra entities, and he argues that of Aristotle's ten categories only substances and qualities correspond to distinct things. The result is the most thorough statement of medieval nominalism.

It is the masterpiece of medieval logic and the clearest example of how a theory of language can be used to economise on what exists.