Confining ourselves for the present to the mental, as distinguished from the physical, side of the operation, we soon find that perception is not so simple a matter as it might at first seem to be.  When a man on a hot day looks at a running stream and “sees” the delicious coolness, it is not difficult to show that he is really performing an act of mental synthesis, or imaginative construction.  To the sense-impression[5] which his eye now gives him, he adds something which past experience has bequeathed to his mind.  In perception, the material of sensation is acted on by the mind, which embodies in its present attitude all the results of its past growth.  Let us look at this process of synthesis a little more closely.

When a sensation arises in the mind, it may, under certain circumstances, go unattended to.  In that case there is no perception.  The sensation floats in the dim outer regions of consciousness as a vague feeling, the real nature and history of which are unknown.  This remark applies not only to the undefined bodily sensations that are always oscillating about the threshold of obscure consciousness, but to the higher sensations connected with the special organs of perception.  The student in optics soon makes the startling discovery that his field of vision has all through his life been haunted with weird shapes which have never troubled the serenity of his mind just because they have never been distinctly attended to.

The immediate result of this process of directing the keen glance of attention to a sensation is to give it greater force and distinctness.  By attending to it we discriminate it from other feelings present and past, and classify it with like sensations previously received.  Thus, if I receive a visual impression of the colour orange, the first consequence of attending to it is to mark it off from other colour-impressions, including those of red and yellow.  And in recognizing the peculiar quality of the impression by applying to it the term orange, I obviously connect it with other similar sensations called by the same name.  If a sensation is perfectly new, there cannot, of course, be this process of classifying, and in this case the closely related operation of discriminating it from other sensations is less exactly performed.  But it is hardly necessary to remark that, in the mind of the adult, under ordinary circumstances, no perfectly new sensation ever occurs.

When the sensation, or complex sensation, is thus defined and recognized, there follows the process of interpretation, by which I mean the taking up of the impression as an element into the complex mental state known as a percept.  Without going into the philosophical question of what this process of synthesis exactly means, I may observe that, by common consent, it takes place to a large extent by help of a reproduction of sensations of various kinds experienced in the past.  That is to say, the details in this act of combination