It is an interesting and suggestive fact in Nature study that at the outset man was thrown utterly upon himself for the very vocabulary of the world-puzzle which presented itself to him for solution. He had not only to unriddle his "inscription in an unknown tongue," but to evolve even the possibility of an explanation out of his inner consciousness. His first theories of the universe were based, not on anything which the cosmos was, independently of him, but upon his own nature and activities as a living animal. This resort to himself as his chief means of interpretation resulted from the very nature of the knowing process; for knowledge of things is never in any absolute sense what things are, but is rather what they are like. When we cognize an object we do it by referring that object to the class of objects which in one or more respects it resembles. And as in this process we draw from the objects most familiar to us the principle of explanation which we need for the less familiar things that have not yet become part of our mental possessions, much depends upon priority in the setting up of mental classes, as well as on the strength of the impression which they make upon the mind. The earliest and deepest class impressions are necessarily those which arise out of man's knowledge of himself--of his body and the parts thereof, of his corporeal activities, and of his feelings and thoughts; next, of the bodies of other men and of their movements; finally, in the order of vividness, of the animate and inanimate objects most nearly related to his life. It is these classes which, by virtue of their priority and strength, naturally acquire dominating influence over all later acquirements, and it is to them that the mind refers the impressions gained from the more remote inorganic world.