Obviously, the astute, enquiring person at once asks, “What makes this stream of thought? To what is its flowing character due?” At first it might seem that one must despair of any satisfactory reply. One feels as did William Harvey, almost hopeless in his quest after the secret of the circulation of the blood. But just as Harvey boldly experimented with his eye on the critical features of his problem, so have a multitude of keen investigators devised test after test to discover the laws of mental phenomena. And while we cannot yet say that “Science has laid her doomful hand” upon all of the intricate secrets of mind, nevertheless, it is becoming more and more certain that the stream of thought is just as much a bodily function as is the breath-stream and the blood-stream. Strangely enough, this conception is not brand-new. It was Aristotle who said, “If the eye were an organism, vision would be its soul.” Similar thinking is revealed in our modern view that mind is a function of the body, and that it depends upon the body for its existence. It “is generated by the body as the result of its immediate contacts with the environment, in much the same way as electricity may be generated by a turbine that has been placed in the midst of a tumbling waterfall.” Moreover, the stream-like character of thought, the play of feeling, the linger and strain of deliberation and expectation are due to the manner in which the storage battery of the brain releases energy, and to the way in which the muscles and glands transform it. William James wrote in 1890, “All consciousness is motor,” that is, it is dependent upon the expression mechanisms of the body; and today we have demonstrated the fundamental rôle of the muscles and glands of the body in “the transformation of the common energies of nature into the special energies of mind.”