The Megarian school of philosophy was founded by Euclides of Megara, one of the pupils of Socrates.

Two main elements went to make up the Megarian as a doctrine. Like the Cynics and the Cyrenaics, Euclides started from the Socratic principle that virtue is knowledge. But into combination with this he brought the Eleatic doctrine of Unity. Perceiving the difficulty of the Socratic dictum he endeavoured to give to the word knowledge a definite context by divorcing it absolutely from the sphere of sense and experience, and confining it to a sort of transcendental dialectic or logic.

The Eleatic unity is Goodness, and is beyond the sphere of sensible apprehension. This goodness, therefore, alone exists; matter, motion, growth and decay are figments of the senses; they have no existence for Reason. Whatever is, is! Knowledge is of ideas and is in conformity with the necessary laws of thought. Hence Plato in the Sophist describes the Megarians as the friends of ideas. Yet the Megarians were by no means in agreement with the Platonic idealism. For they held that ideas, though eternal and immovable, have neither life nor action nor movement.