The emotion or feeling of self may be regarded as an instinct, doubtless evolved in connection with its important function in stimulating and unifying the special activities of individuals.[38] It is thus very profoundly rooted in the history of the human race and apparently indispensable to any plan of life at all similar to ours. It seems to exist in a vague though vigorous form at the birth of each individual, and, like other instinctive ideas or germs of ideas, to be defined and developed by experience, becoming associated, or rather incorporated, with muscular, visual and other sensations; with perceptions, apperceptions and conceptions of every degree of complexity and of infinite variety of content; and, especially, with personal ideas. Meantime the feeling itself does not remain unaltered, but undergoes differentiation and refinement just as does any other sort of crude innate feeling. Thus, while retaining under every phase its characteristic tone or flavor, it breaks up into innumerable self-sentiments. And concrete self-feeling, as it exists in mature persons, is a whole made up of these various sentiments, along with a good deal of primitive emotion not thus broken up. It partakes fully of the general development of the mind, but never loses that peculiar gusto of appropriation that causes us to name a thought with a first-personal pronoun. The other contents of the self-idea are of little use, apparently, in defining it, because they are so extremely various. It would be no more futile, it seems to me, to attempt to define fear by enumerating the things that people are afraid of, than to attempt to define “I” by enumerating the objects with which the word is associated. Very much as fear means primarily a state of feeling, or its expression, and not darkness, fire, lions, snakes, or other things that excite it, so “I” means primarily self-feeling, or its expression, and not body, clothes, treasures, ambition, honors, and the like, with which this feeling may be connected. In either case it is possible and useful to go behind the feeling and enquire what ideas arouse it and why they do so, but this is in a sense a secondary investigation.