The three classic laws of thought are attributed to Aristotle and are fundamental in the field of logic. The three laws are:

- The
*law of identity*, which states that a thing is identical with itself. - The
*law of noncontradiction*, which states that two contradictory statements cannot be true at the same time. - The
*law of excluded middle*, which states that, for any proposition, either the proposition is true or its negation is true.

The last two laws form the basis of the technique of *reductio ad absurdum*, as well as being fundamental to the study of propositional logic.