Philosophical logic has been defined by Jonathan Lowe "as the philosophical elucidation of those notions that are indispensable for the proper characterisation of rational thought and its contents".

The notions in question are ones like those of reference, predication, identity, truth, negation, quantification, existence, necessity, definition and entailment.

Philosophical logic is not concerned with the psychological processes connected with thought, or with emotions, images and the like. It is concerned only with ideas that are capable of being true and false. Gottlob Frege is regarded by many as the founder of modern philosophical logic.