Law of thought

In logic and philosophy, "laws of thought" is a dated expression referring to three logical principles: the law of identity (LOI), the law of non-contradiction (LNC), and the law of excluded middle (LEM).

The expression "laws of thought" gained prominence through its use by idealist and conceptualist logicians such as George Boole (1815–1864). Boole named his second logic book An Investigation of the Laws of Thought on Which are Founded the Mathematical Theories of Logic and Probabilities (1854).

Among contemporary logicians, the expression "laws of thought" is widely regarded as obsolete. The reasons for this can be summarized as follows:

1. The laws have no foundational status. They are tautologies or logical truths of classical logic, but are not typically taken to be axioms, but rather theorems. No viable system of logic can be constructed in which the "three laws" would be the only axioms.
2. Following the rejection of psychologism in the 19th century by some authors (e.g., Frege, Husserl) psychology and logic are considered to be separate disciplines. The study of how people think and reason belongs to cognitive psychology. Logic is concerned with the relationships of logical consequence between propositions or sentences.
3. The laws are not universally accepted. There are non-classical logics that reject one or more of the three principles.

While there are some contemporary philosophers of logic who consider that logical laws can be regarded as constitutive of rational thought, they too reject the old view of logic as the "three laws".