PRIMARY QUALITIES. — These are, according to the common usage after Locke (q.v.), the qualities (q.v.) of bodies without which the body itself would be inconceivable and which are therefore held to exist really and physically in material reality. The primary qualities are extension, shape, number, local mode, and to these are opposed the secondary qualities, such as colours, sounds, tastes, odours, which are considered “subjective.”
The principle of the distinction between primary and secondary qualities can already be found in Democritus (q.v.), who affirmed: “Opinion is colour, opinion is sweet, opinion is bitter; truth is atoms and void” (fr. 49). In the modern era, Galileo, in the famous passage of *Il Saggiatore* (Opere, ed. naz., VI, p. 348), states that shape, size, number and local motion necessarily belong to the body (and are those which Locke, in the *Essay*, II, ch. 8, § 9, will call primary qualities), whereas colour, sound, taste and odour reside in the sensitive body. For Descartes too (*Meditations*, II, ed. Adam-Tannery, VII, p. 30 ff.) the only real quality of bodies is extension. This doctrine is accepted by those who profess the so-called critical realism (q.V. PERCEPTIONISM; REALISM).