Full Idea
Aristotle identified four 'figures' of argument, based on combinations of Subject (S) and Predicate (P) and Middle term (M). The addition of 'all' and 'some', and 'has' and 'has not' got the property, resulted in 256 possible syllogisms, 19 of them valid.
Gist of Idea
Aristotelian identified 256 possible syllogisms, saying that 19 are valid
Source
report of Aristotle (Prior Analytics [c.328 BCE]) by Keith Devlin - Goodbye Descartes Ch.2
Book Reference
Devlin,Keith: 'Goodbye Descartes: the end of logic' [Wiley 1997], p.42
A Reaction
[Compressed version of Devlin] What Aristotle did was astonishing, and must be one of the key ideas of western civilization, even though a lot of his assumptions have been revised or rejected.