Reid never doubted that sensations are an ingredient of perception. The having of sensations is not sufficient for perception, however. In Reid's words, “if nature had given us nothing more than impressions made upon the body, and sensations in our minds corresponding to them, we should in that case have been merely sentient, but not percipient beings.” Hinting at his own analysis of perception, Reid continues: “We should never have been able to form a conception of any external object, far less a belief of its existence” (IHM VI, xxi [187b; B 176]). As we saw earlier, Reid is convinced that “All the systems of philosophers about our senses and their objects have split upon this rock, of not distinguishing properly sensations which can have no existence but when they are felt, from the things suggested by them” (IHM V, viii [130b–131a; B 72–3]).

Perception occurs when one's environment is represented to one as being a certain way – perceptually represented, of course.

It occurs when one's experience is objectivated – to use terminology from Kant's Prolegomena. In perception one leaves behind the confines of one's own mind and brings one's spatial environment into the picture. One brings it into mind not by drawing inferences about it but by representing it as being a certain way – or better by its being represented to one as being a certain way.