PRIMARY QUALITIES. - According LOCKE, JOHN (v.), these are the qualities (v.) of bodies without which the body itself would be inconceivable, and which are therefore held to exist really and physically in material reality. Such are extension, figure, number, and local motion; opposed to them are the secondary qualities, such as colours, sounds, tastes, and odours, which are held to be “subjective.”
The principle of the distinction between primary and secondary qualities Democrito (v.), who stated: “Colour is opinion, sweet is opinion, bitter is opinion; truth is atoms and the void” (frg. 49). In the modern age Galileo, in the famous passage of the Saggiatore (Opere, ed. naz., VI, p. 348), affirms that figure, magnitude, number, and local motion necessarily belong to the body (and are those which Locke, in the Essay, II, cap. 8, § 9, will call primary qualities), whereas colour, sound, taste, and odour “have their abode in the sentient body.” For Cartesio likewise (Medit., II, ed. Adam-Tannery, VII, p. 30 sgg.), the sole real quality of bodies is extension. This doctrine is accepted by those who profess the so-called critical realism (v. PERCEZIONISMO; REALISMO).