It lies in the nature of this mechanism that the subjective image is not identical with the objective original, but is modified and even distorted by the panes through which it penetrates to the inner being of the organism. What the subject perceives is never anything but a symbol of the object, never the object itself; but this symbol suffices to enable the consciousness to form an idea of the object, just as letters enable the reader to take in words and thoughts. We must conceive the development of consciousness to go hand in hand with that of the senses. The more windows the organism can open to the outer world the more easily and the more clearly does its image penetrate. The number of objects which the subject can take in is the measure of the perfection of its consciousness. The protista, lacking specific sense organs and possessing only the general sensitiveness of protoplasm, can form only to a very limited extent and with very little variety an inner realization of the stimuli of the outer world. Its consciousness is necessarily very restricted and exceedingly dim. Consciousness is enlarged and grows clearer as the organism develops and its general sensitiveness is differentiated into specific senses, until we reach the level of man whose consciousness embraces far more of the outer world than does that of any other living creature; because, lacking new senses, he has succeeded in amplifying and enlarging those he possesses, and has by artificial means made himself capable of perceiving stimuli to which he is not directly susceptible and which therefore would have remained unknown to him; to a certain extent he has translated them into a form which his senses can perceive.
I do not overlook any of the difficulties which my attempt to explain consciousness leaves untouched. On all sides the most urgent and disquieting questions arise. Above all, the fundamental question, the most enigmatic of all: how is an external stimulus, that is a movement, a vibration, converted into a sensation, a perception? Further: must we in the consciousness distinguish between the frame and its contents, the conceptual mechanism and the concept? Or do the two coincide? Is there no consciousness without a conceptual content? And is it the movement entering into the organism, the inner realization of the outer world which, transmuting itself in an incomprehensible manner into a concept, creates consciousness, becomes consciousness? Is the consciousness of the man standing upon the highest plane of intellectuality the greatest consciousness possible? Does there exist anywhere in the universe a more abundant, perhaps an infinitely more abundant consciousness than that of human beings on the Earth, and will the latter ever rise to this height? It is obvious that a development is in progress. There was a time when the most comprehensive, the clearest consciousness on earth was that of the trilobite or the cephalopod. Evolution has gone as far as man. Does it stop at that or will it continue?