Logic

Summa Logicae

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William of Ockham

1323

The Summa Logicae is the most comprehensive treatise on logic produced in the medieval period and one of the most thorough expositions of term logic ever written. Ockham composed it as a teaching manual for students in the Franciscan schools, but it transcended that occasion to become a foundational document of late medieval philosophy. Divided into three parts — on terms, on propositions, and on arguments — it systematically develops a theory of language based on the distinction between mental, spoken, and written signs, argues for a strictly nominalist account of universals (only individual things exist; universal terms are signs that stand for many things at once), and demolishes the realist positions of Aquinas and Scotus on the reality of universals and relations. Ockham's razor — the principle that entities should not be multiplied beyond necessity — finds its philosophical home here, applied relentlessly to strip ontology down to what logic actually requires.