Now, if it be true that the consciousness aroused by objects around us, through sensation, is an accompaniment of certain physiological changes in the brain, it is clear that the localization of their points of origin in special parts of the skin, and the outward projection of the objects exciting vision, is an act of the mind quite distinct from the mere passive response in consciousness which we call an impression, and more complex than that mental activity which, through discrimination and recognition, converts the bare impression into a sensation. It is, in fact, part of that mental process which is called _perception_.[FP] Sensation has nothing to do with the objects around us as such; it is by perception that we are aware of their existence. Let us now follow the process of perception a little further, always remembering that it involves certain activities of the mind. These activities are too often ignored. We often speak of the senses as the avenues of knowledge, and John Bunyan, likening the soul to a citadel, spoke of the five gateways of knowledge, Eye-gate, Ear-gate, Mouth-gate, Smell-gate, and Feel-gate. Hence arises a vague notion that through the eye-gate, for example, a sort of picture of the external object somehow enters the mind. And this idea is no doubt fostered by the fact that an inverted image of the object is formed on the retina, though how the inverted image is turned right way up again in passing into the mind bothers some people not a little.[FQ] A much closer analogy is this: Something stands without and knocks at the doorway of sense, and from the nature of the knocks we learn somewhat concerning that which knocks. In other words, at the bidding of certain stimuli from without we construct that mental product which we call the object of sense. It is of these mental constructions--"_constructs_"[FR] I will call them for convenience--that I have now to speak.