Against his will, however, Locke was drawn to enlarge the subjective sphere. The actual existence of mind was evident: you had only to notice the fact that you were thinking. Conscious mind, being thus known to exist directly and independently of the body, was a primary constituent of reality: it was a fact on its own account.[6] Common sense seemed to testify to this, not only when confronted with the "I think, therefore I am" of Descartes, but whenever a thought produced an action. Since mind and body interacted,[7] each must be as real as the other and, as it were, on the same plane of being. Locke, like a good Protestant, felt the right of the conscious inner man to assert himself: and when he looked into his own mind, he found nothing to define this mind except the ideas which occupied it. The existence which he was so sure of in himself was therefore the existence of his ideas. Here, by an insensible shift in the meaning of the word "idea", a momentous revolution had taken place in psychology. Ideas had originally meant objective terms distinguished in thought-images, qualities, concepts, propositions. But now ideas began to mean living thoughts, moments or states of consciousness. They became atoms of mind, constituents of experience, very much as material atoms were conceived to be constituents of natural objects. Sensations became the only objects of sensation, and ideas the only objects of ideas; so that the material world was rendered superfluous, and the only scientific problem was now to construct a universe in terms of analytic psychology. Locke himself did not go so far, and continued to assign physical causes and physical objects to some, at least, of his mental units; and indeed sensations and ideas could not very well have other than physical causes, the existence of which this new psychology was soon to deny: so that about the origin of its data it was afterwards compelled to preserve a discreet silence. But as to their combinations and reappearances, it was able to invoke the principle of association: a thread on which many shrewd observations may be strung, but which also, when pressed, appears to be nothing but a verbal mask for organic habits in matter.