Observed _facts_ can be recalled, and this is the typically human form of recall. In animals, we see the recall of tendencies and of learned movements, but no clear evidence of the recall of observed facts. To be recalled with certainty, a fact must have been definitely noted when it was before us. If we have definitely noted the color of a person's eyes, we are in a position to testify that his eyes are brown, for example; otherwise, we may say that we think probably his eyes are brown; because we have certainly noticed that he is dark, and the dark eyes fit best into this total impression. We say that a fact is recalled when we think of it without its being present to the senses. While the original {368} observation of the fact was a response to a sensory stimulus, the recall of it is a response to some other stimulus, some "substitute stimulus". When John is before me, I observe that his eyes are brown in response to a visual stimulus; but I later recall this fact in response simply to the name "John", or in response to the question as to what is the color of John's eyes. I see what a square is by seeing squares and handling them, and later I get this idea simply in response to the word "square" in conversation or reading. Memory Images Now, can _sensations_ be recalled, can they be aroused except by their natural sensory stimuli? Can you recall the color blue, or the sound of a bugle, or the odor of camphor, or the feel of a lump of ice held in the hand? Almost every one will reply "Yes" to some at least of these questions. One may have a vivid picture of a scene before the "mind's eye", and another a realistic sound in the "mind's ear", and they may report that the recalled experience seems essentially the same as the original sensation. Therefore, sensory reactions are no exception to the rule of recall by a substitute stimulus. A sensation or complex of sensations recalled by a substitute stimulus is called a "mental image" or a "memory image". Individuals seem to differ in the vividness or realism of their memory images--the likeness of the image to an actual sensation--more than in any other respect. Galton, in taking a sort of census of mental imagery, asked many persons to call up the appearance of their breakfast table as they had sat down to it that morning, and to observe how lifelike the image was, how complete, how adequate in respect to color, how steady and lasting, and to compare {369} the image in these respects with the sensory experience aroused by the actual presence of the scene. Some individuals reported that the image was "in all respects the same as an original sensation", while others denied that they got anything at all in the way of recalled sensation, though they could perfectly well recall definite facts that they had observed regarding the breakfast table. The majority of people gave testimony intermediate between these extremes.