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Rh But there was another and very important step made by Descartes, in restricting the ideal Theory to the primary qualities of matter; its secondary qualities (of colour, sound, smell, taste, heat, and cold) having, according to him, no more resemblance to the sensations by means of which they are perceived, than arbitrary sounds have to the things they denote, or the edge of a sword to the pain it may occasion. (Princ. Pars iv. §§ 197, 198.) To this doctrine he frequently recurs in other parts of his works.

In these modifications of the Aristotelian Theory of Perception Locke acquiesced entirely; explicitly asserting, that “the ideas of primary qualities are resemblances of them, but that the ideas of secondary qualities have no resemblance to them at all.” Essay, B. ii. c. viii. § 15.

When pressed by Gassendi to explain how images of extension and figure can exist in an unextended mind, Descartes expresses himself thus : “”, § 4.

In this reply it is manifestly assumed as an indisputable principle, that the immediate objects of our thoughts, when we imagine or conceive the primary qualities of extension and figure, are ideas or species of these qualities; and, of consequence, are themselves extended and figured. Had it only occurred to him to apply (mutatis mutandis) to the perception of primary qualities his own account of the perception of secondary qualities (that it is obtained, to wit, by the media of sensations more analogous to arbitrary signs, than to stamps or pictures), he might have eluded the difficulty started by Gassendi, without being reduced to the disagreeable necessity of supposing his ideas or images to exist in the brain, and not in the mind. The language of Mr Locke, it is observable, sometimes implies the one of these hypotheses, and sometimes the other.

It was plainly with the view of escaping from the dilemma proposed by Gassendi to Descartes, that Newton and Clarke were led to adopt a mode of speaking concerning perception, approaching very nearly to the language of Descartes. “Is not,” says Newton, “the sensorium of animals the place where the sentient substance is present; and to which the sensible species of things are brought, through the nerves and brain, that there they may be perceived by the mind present in that place?” And still more confidently Dr Clarke: “Without being present to the images of the things perceived, the soul could not possibly perceive them. A living substance can only there perceive where it is present. Nothing can any more act or be acted upon where it is not present, than it can when it is not.” The distinction between primary and secondary qualities was afterwards rejected by Berkeley, in the course of his argument against the existence of matter; but he continued to retain the language of Descartes concerning ideas, and to consider them as the immediate, or rather as the only objects of our thoughts, wherever the external senses are concerned. Mr Hume’s notions and expressions on the subject are very nearly the same.

I thought it necessary to enter into these details, in order to shew with what limitations the remark quoted from Dr Reid in the beginning of this note ought to be received. It is certainly true, that the Cartesian system may be said to form the ground-work of Locke’s Theory of Perception, as well as of the sceptical conclusions deduced from it by Berkeley and Hume; but it is not the less true, that it forms also the ground-work of all that has since been done towards the substitution, in place of this scepticism, of a more solid fabric of metaphysical science.