So far, Dewey has been concerned with the nature of meaning. He now turns to knowledge, and the knowing process as that which gives meaning to experience. Knowledge, or science, he says, is a process of following out the ideal element in experience. "The idealisation of science is simply a further development of this ideal element. It is, in short, only rendering explicit and definite the meaning, the idea, already contained in perception."[30] But if perception is already organized by thought, the sensations must have been related in a 'productive imagination.' Dewey, however, does not recognize such a necessity. The factor of meaning is ideal, he continues, because it is not present as so much immediate content, but is present as symbolized or mediated. But the question may be asked, "Whence come the ideal elements which give to experience its meaning?" No answer can be given except by psychology, as an inquiry into the facts, as contrasted with the logical necessity of experience. Sensations acquire meaning through being identified with and discriminated from other sensations to which they are related. But it is not as mere existences that they are compared and related, but as already ideas or meanings. "The identification is of the meaning of the present sensation with some meaning previously experienced, but which, although previously experienced, still exists because it _is_ meaning, and not occurrence."[31] The existences to which meanings attach come and go, and are new for every new appearance of the idea in consciousness; but the meanings remain. "The experience, as an existence at a given time, has forever vanished. Its meaning, as an ideal quality, remains as long as the mind does. Indeed, its remaining is the remaining of the mind; the conservation of the ideal quality of experience is what makes the mind a permanence."[32] It is not possible, Dewey says, to imagine a primitive state in which unmeaning sensations existed alone. Meaning cannot arise out of that which has no meaning. "Sensations cannot revive each other except as members of one whole of meaning; and even if they could, we should have no beginning of significant experience. Significance, meaning, must be already there. Intelligence, in short, is the one indispensable condition of intelligent experience."[33]