Behind and beneath all these considerations we should never lose sight of the great main facts--that thought is an activity; that its function therefore is to represent or reproduce our pure exertional activity; that such representation is _at the basis_ of all our concepts of externality; that sensation, _per se_ is mere interruption of activity; that _per se_ it possesses no spatial or extensive or external suggestiveness; that sensations nevertheless serve to denote or give feature and particularity to our experience of activity; that all perception of the external is at bottom therefore a mental representation of exertional activity and its forms, denoted, punctuated, identified by sensation, which latter by itself, we repeat, carries no suggestion of externality. This view revolutionises the whole psychology of Perception, and therefore, though it at once gives to that science a much-needed unity, clarity, and simplicity, it will naturally be accepted with reluctance by the laborious authors of the cumbrous theories still generally current. FOOTNOTES: [18:1] His reason is that we _ab origine_ localise sensations with reference to our organism. This, of course, means by reference to the system of potent energy in which our organism essentially consists. III THE TWO TYPICAL THEORIES OF KNOWLEDGE The evolution of living organisms is in general a gradual and continuous process. But it is nevertheless true that it presents well-marked stages and can best be described by reference to these. Frequently, moreover, the meaning and true nature of the movement at one stage is only revealed after a subsequent stage has been reached. The development of a brain or cerebrum marks one important advance. The presence of this organ renders possible to the animal in varying degree what are called representations of objects, and the faculty of making such representations appears to be a condition precedent to the development of deliberation, volition, and purposive action as opposed to reflex or instinctive activity. The latter is specially characteristic of other orders of organic existence such as the Articulata--being remarkably exemplified in the activities of the social insects such as the bee.