An Inquiry into the Human Mind on the Principles of Common Sense

A treatise on the five senses that opens Reid's attack on what he calls the 'ideal system', the doctrine running from Descartes through Locke to Berkeley and Hume that we perceive only ideas in our own minds. Reid argues that this doctrine leads straight to Hume's scepticism, and that the fault lies in the premise, not in ordinary belief. After an introduction, he examines smelling, tasting, hearing, touch and seeing in turn. He separates sensation, a feeling with no object beyond itself, from perception, which carries an immediate belief in an external object. Sensations work as natural signs: by our constitution they suggest qualities such as hardness or extension that they in no way resemble. The long chapter on seeing distinguishes original from acquired perception and includes his 'geometry of visibles'. Throughout, Reid holds that certain first principles, the principles of common sense, are part of our constitution and need no proof.

Reid's direct realism and his defence of common sense against sceptical philosophy have returned to the centre of debates about perception and knowledge.