Origins: Quod Erat Demonstrandum

When did people begin to think logically? There is no answer to this question. Rationality of some kind existed before even the earliest stone tools, since such objects must have been produced in the rational expectation that they would make certain tasks easier. As far back as we can imagine, human beings behaved rationally (which is not to say that they haven’t also behaved irrationally): It has always been “logical” to suppose that the sun would indeed rise again, that a rock thrown upward would fall to the ground, and that briny seawater would not make a very healthy drink.

Systematic or formal logic, on the other hand, is a different matter. It was invented by Aristotle (384–322 B.C.) in Athens, and recorded in a group of writings known as the Organon. In one of these works, the Prior Analytics, Aristotle attempted to provide a complete analysis of the valid forms of reasoning. His attempt was so successful that it remained the dominant strain in logic until the mid-19th century and is still taught in some universities today.