It is the same with all minds with regard to the objects and scenes which happen to be of special interest. The following illustration will serve to make the matter clearer to readers who are not accustomed to pay attention to their own mental processes. When any common object, such as a chair, or spade, or apple, is thought of or spoken of, an image of a picture of it instantly comes before the mind's eye; not of a particular spade or apple, but of a type representing the object which exists in the mind ready for use on all occasions. With the question of the origin of this type, this spade or apple of the mind, we need not concern ourselves here. If the object thought or spoken of be an animal--a horse let us say, the image seen in the mind will in most cases be as in the foregoing case a type existing in the mind and not of an individual. But if a person is keenly interested in horses generally, and is a rider and has owned and loved many horses, the image of some particular one which he has known or has looked at with appreciative eyes will come to mind; and he will also be able to call up the images of dozens or of scores of horses he has known or seen in the same way. If on the other hand we think of a rat, we see not any individual but a type, because we have no interest in or no special feeling with regard to such a creature, and all the successive images we receive of it become merged in one--the type which already existed in the mind and was probably formed very early in life. With the dog for subject the case is different: dogs are more with us--we know them intimately and have perhaps regarded many individuals with affection; hence the image that rises in the mind is as a rule of some dog we have known.
The important point to be noted is, that while each and everything we see registers an impression in the brain, and may be recalled several minutes, or hours, or even days afterwards, the only permanent impressions are of the sights which we have viewed emotionally. We may remember that we have seen a thousand things in which at some later period an interest has been born in the mind, when it would be greatly to our pleasure and even profit to recover their images, and we strive and ransack our brains to do so, but all in vain: they have been lost for ever because we happened not to be interested in the originals, but viewed them with indifference, or unemotionally.