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.

The fact is that there are two sorts of unobjectionable psychology, neither of which describes a mechanism of disembodied mental states, such as the followers of Locke developed into modern idealism, to the confusion of common sense.[8] One unobjectionable sort of psychology is biological, and studies life from the outside.  The other sort, relying on memory and dramatic imagination, reproduces life from the inside, and is literary.  If the literary psychologist is a man of genius, by the clearness and range of his memory, by quickness of sympathy and power of suggestion, he may come very near to the truth of experience, as it has been or might be unrolled in a human being.[9] The ideas with which Locke operates are simply high lights picked out by attention in this nebulous continuum, and identified by names.  Ideas, in the original ideal sense of the word, are indeed the only definite terms which attention can discriminate and rest upon; but the unity of these units is specious, not existential.  If ideas were not logical or aesthetic essences but self-subsisting feelings, each knowing itself, they would be insulated for ever; no spirit could ever survey, recognise, or compare them; and mind would have disappeared in the analysis of mind.