Whatever philosophical position we adopt, it is a practically indubitable fact of experience that the manifestation of a higher kind of intellectual life is bound up with the functioning of a highly developed brain. The world that we construct out of our sensations and perceptions, and which we always comfortably think of as being quite simply there, is not in fact manifest just by existing; in order to be so, it requires very special events in very special parts of itself, namely the functions of a brain. This is an extraordinarily remarkable state of affairs, in view of which one cannot help asking, however tentatively, what are the special distinguishing characteristics of events in the brain, by which they and they alone bring about this manifestation; is it possible to state, or at least to conjecture, which material events have this capacity, and which have not? Or, more simply, and perhaps just as unambiguously, which material events are directly connected with consciousness?

For the present-day thinker with a rational-scientific outlook there is a quick and obvious answer to this question: to judge by our own experience and the analogy provided by the higher animals, consciousness is associated exclusively with a certain type of event in organic, living matter, that is, with certain nerve functions. As to how far down the ranks of animals we can suppose that consciousness extends, and as to how it is constituted in its earliest stages, the search for more definite ideas on this subject would set us an insoluble and superfluous task which—we are told—we may well leave to dreamers with nothing better to do.