he would recognize them instantly, and ever afterward the impression which they produce on sight would be associated with the impression which they produce on touch, and he would know them when he saw them. That is the way in which the perception of a particular object is formed—by the association of all the sensations which it is adapted to excite in our different senses, their combination in what we call an idea. For example, in the idea of an orange are combined the sensations which we get by tasting it, by touching it, by smelling it, by looking at it, by handling it, each sensation having been acquired by its particular sense in the course of an education which has been going on ever since we were born; when we have got them in that way, they combine to form the idea of the orange; and it is by virtue of this idea, which has been formed and registered in the mind, that we are able to think of an orange, that is, to form a mental image of it, when it is not present to any sense, and to recognize it instantly when it is. It is plain, then, how large a part, by virtue of its past experience, the mind contributes to each perception; when we look at an orange it tacitly supplies to the impression which it makes on sight all the information about it which we have got at different times by our other senses, and which sight does not in the least give us; the visual impression is no more in truth than a sign to which experience has taught us to give its proper meaning, just as the written or spoken word in any language is a sign which is meaningless until we have been taught what to mean by it. So true it is that the eye only sees what it brings the faculty of seeing, and that many persons have eyes, yet see not.
This being so, it is clear that the idea in the mind will very much affect the perception, and that if any one goes to look at something, or to taste something, or to feel something, with a strongly-preconceived idea of what it is, he will be likely, if it is not what he thinks it, to have a mistaken perception—to see, or feel, or touch, what he thinks it is, not what it really is. This is, indeed, one of the most common causes of erroneous observation, and one which the scientific observer knows well he must always vigilantly guard against. If a man has a foregone conclusion of what he will see, it is not safe to trust his observation implicitly, either in science or in common life. We witness the most striking examples of this dominion of the idea over sense in persons who have been put into the so-called mesmeric state. The operator gives them simple water to taste, telling them at the same time that it is some nauseating and bitter mixture, and they spit it out with grimaces of disgust when they attempt to drink it; when he tells them what he offers them is sweet and pleasant, though it is as bitter as wormwood, they smack their lips as if they had tasted something remarkably good; if assured that a swarm of bees is buzzing about them, they are in the greatest trepidation, and go through violent antics to beat them off. Their senses are dominated by the idea suggested, and they are very much in the position of an insane