Jean-Paul Sartre, in his book The Imaginary, describes a mental image of a chair as follows:
"My perception is, in accordance with the terminology that we have adopted, a certain consciousness and the chair is the object of that consciousness. Now I close my eyes and I produce the image of the chair that I have just perceived." (p. 7)
After saying this, he goes on to assert that his use of the word "image" is primarily in keeping with the traditional use of the word in philosophy. However, in order to avoid ambiguity, he believes that it is better to speak of it as a relation:
"The word 'image' could only indicate therefore the relation of consciousness to the object; in other words, it is a certain way in which the object appears to consciousness, or, if one prefers, a certain way in which consciousness presents to itself an object." (p. 7)
According to Jonathan Webber, who wrote the introduction to the book, this is not simply a question of terminology. Rather, it represents Sartre's stand against the traditional philosophical position:
"This is probably because in the three hundred years preceding the publication of The Imaginary, perception itself was generally understood as involving mental pictures. On such a view, to say that mental imagery consists in contemplating mental pictures is to say that it is akin to perceptual experience, and vice versa. This is the view that Sartre calls 'the illusion of immanence'." (p. xx)
The following might help to clarify Sarte's position:
"Now – this is, above all, what reflection teaches us – whether I perceive or imagine that chair, the object of my perception and that of my image are identical: it is that straw-bottomed chair on which I sit. It is simply that consciousness is related to this same chair in two different ways. In both cases, it aims at the chair in its concrete individuality, in its corporeality. Only, in one of the cases, the chair is 'encountered' by consciousness; in the other, it is not. But the chair is not in consciousness. Not even as an image." (p. 7)
First of all, it seems to me that what qualifies something as being an image is that it looks like an image. Sartre closed his eyes and could presumably see a extendend figure with contours, edges and perhaps color. He does, for example, give a very vivid description of hypnagogic images:
"It is a question, indeed, of a world in perpetual motion: figures are transformed, in rapid succession, a line becomes a string, a string becomes a face, etc. In addition, each figure is animated by translation movements and rotations, which are but whirling wheels of fire, shooting stars that descend quickly, faces that approach or recede." (p. 47)
Secondly, the fact that he closed his eyes seems to indicate that the relation between conciousness and its object was broken. Although it might be claimed that an intentional relation remains, the mechanics of closing the eyes seems to be a very definitive way to break any visual relation.
Because of these considerations, his aversion to the use of the word "image" seems rather baffling. If the perception of something with all the characteristics of an image is, in fact, not an image at all but a relation, couldn't we equally conclude that there really is no such thing as an image? What could the word "image" possibly mean if it requires more than appearing as an image?
Perhaps a more important question is: What does Sartre gain by this? If mental images have all the characteristics of images, in what way is he really differentiating his position from the traditional view?