If indeed imagination is that in virtue of which we say that an image comes about to us and not as when we say something metaphorically, then it is some kind of faculty or capacity, by virtue of which we judge and say the truth or the untruth. Such are perception, belief, scientific knowledge, and intellect.
It is not a perception
That imagination is not actually a perception is clear from the following. For perception is either a faculty or an activity, as sight and seeing are, while there are certain things that appear to us even when neither of these two is present, just as whatever happens to us during our sleep. Moreover, perception is always present, imagination not. But if these two were the same in respect of the activity, then it would be possible for all animals to imagine; but it seems that this is not happening, as it is with the ant or bee or grub. Next, perceptions are always true, while imaginations are for the most part false. Furthermore, we do not say, when we act on a perceptible object accurately, that this object does not seem to us to be a man; rather when we do not have a distinct perception of it, and then it is either true or false. And just as we were saying before, even with our eyes closed visions appear to us too.
But indeed imagination will be none of these faculties that are always true, e.g. knowledge or intellect; for there is false imagination too. There remains, then, to see if it is belief; for there comes about both true and false belief. But faith follows on belief (for we cannot believe things without having faith of them), and no animal has faith though many do have imagination. Moreover, every belief is accompanied by faith, faith by conviction, conviction by reasoning; whereas some animals do have imagination, but no reasoning.
It is clear, therefore, that imagination must be neither belief together with perception, nor through perception, nor a mixture of belief and perception, both for that reason and because it is obvious that we have no belief of any other thing except of that of which we have also its sensation. What I mean is that the complex made of the belief in white and the perception of white is imagination; but certainly not that one of the belief in the good and the perception of white. Imagination, then, is to believe exactly what one perceives and not incidentally.
However, imagination occurs also for things that are false, and about which our supposition is at the same time true; for example, the sun seems to be a foot in length thought we are convinced that it is bigger than the earth. So it follows that either we have abandoned the true belief that we had, although the case remains as it is, without having forgotten it or been persuaded to the contrary, or, if we retain the same belief, this must needs be both true and false. But the case was false whenever we did not notice the change of the object. Imagination, then, is not any one of the foregoing nor is it composed of them.
But since it is possible for one thing when moved to provoke movement to another one, and imagination seems to be a kind of movement and not to occur without sensation but belong to things that perceive and has to do with things of which there is perception, and since the activity of perception may provoke movement, and that movement is necessarily identical with the perception, it could be possible for that movement neither to occur without sensation nor to belong to those that cannot perceive, and what possesses it to act and be acted upon many things in accordance with it, and such as to be both true and false.
This happens for the following reasons: The perception of the peculiar objects for each one sensation is true or false to the least possible degree. Secondly, the objects of perception refer to those which exist incidentally; and here at once it is possible for one to be deceived, for we are not mistaken about this whether it is white or not, but we are mistaken about whether the white object before us is this one or that one. Thirdly, out of the objects of perception which are common to all senses and follow upon the incidentals, and to which the peculiar objects belong; I mean, for example, movement and magnitude, which supervene on the objects of perception, and about which one may be at once deceived to a great extend by the sense.
The movement that comes from the activity of perception will differ from the movement that comes from these three kinds of perception (that is, the perception of the peculiar, the incidental, and the common objects). And the first kind (that is, the movement that takes place together with the activity of the perception of the peculiar objects) is true as long as perception is present, while the other two (that is, the movements that come from the perceptions of the incidental and common objects), whether perception is present or not, will be false, and especially when the object of perception is too far.
If, then, nothing else has the mentioned characteristics except imagination, and it is what we said, imagination must be a movement that comes about out of the actual sensation. And since sight is the main sense we have, imagination (phantasia) derives its name from light (phaos), because without light we cannot see. And on the ground that imaginations persist and are similar to sensations, animals do many things on account of them, some because they have no mind, like all other animals, others because sometimes their mind is obscured by some passion, or disease, or sleep, like men.
As for imagination, then, what it is, and why it is, enough has been said so far.
Bibliography: Aristotle De anima (428a.1 to 429a.9)
Translation: George Kotsalis