mary and secondary qualities of matter. The doctrine, as you know, is this, that the primary qualities are extension, figure, and solidity; that these exist objectively in the things themselves, and that we have a direct perception of them as they thus exist; while again the secondary qualities, such as heat and cold, colours and sounds, tastes and smell, are subjective affections existing merely in us. These are not properly the qualities of matter, but are rather the names of our sensations. The difference, however, between this doctrine and that of the Atomists consists in this circumstance, that while the modern propounders of the doctrine have held that there were certain occult qualities in matter corresponding to our sensations of heat, colour, taste, smell, and so forth—occult qualities by which these sensations are induced—the Atomists had recourse to no such hypothesis. They conceived that the nature of the atoms, which has been already explained as consisting in differences of shape and arrangement—they conceived that this was quite sufficient of itself to account for the variety of our sensations, and accordingly the hypothesis of occult qualities really existing in material things, and inducing our sensations, formed no part of their system. Our sensations were explained on mechanical and quantitative grounds as resulting from the different shapes and degrees of solidity in the atoms by which our organs of sense were affected. The Atomic theory of sensation and perception was thus considerably simpler than the doctrine
