It may, however, still be said that the process of direct adaptation must tend to establish such a consensus of true belief.  Now, I do not wish for a moment to dispute that the growth of intelligence by the continual exercise of its functions tends to such a consensus:  this is assumed to be the case by everybody.  What I want to point out is that there is no scientific proof of this position.

The correspondence of internal to external relations is obviously limited by the modes of action of the environment on the organism, consequently by the structure of the organism itself.  Scientific men are familiar with the idea that there may be forces in the environment which are practically inoperative on the organism, there being no corresponding mode of sensibility.  And even if it be said that our present knowledge of the material world, including the doctrine of the conservation of energy, enables us to assert that there is no mode of force wholly unknown to us, it can still be contended that the environment may, for aught we know, be vastly more than the forces of which, owing to the nature of our organism, we know it to be composed.  In short, since, on the evolution theory viewed as a scientific doctrine, the real external world does not directly mirror itself in our minds, but only indirectly brings our perceptions and representations into adjustment by bringing into adjustment the nervous organism with which they are somehow connected, it is plain that we cannot be certain of adequately apprehending the external reality which is here assumed to exist.

Science, then, cannot prove, but must assume the coincidence between permanent common intuitions and objective reality.  To raise the question whether this coincidence is perfect or imperfect, whether all common intuitions known to be persistent are true or whether there are any that are illusory, is to pass beyond the scientific point of view to another, namely, the philosophic.  Thus, our study of illusion naturally carries us on from scientific to philosophic reflection.  Let me try to make this still more clear.

Transition to Philosophic View.

All science makes certain assumptions which it never examines.  Thus, the physicist assumes that when we experience a sensation we are acted on by some pre-existing external object which is the cause, or at least one condition, of the sensation.  While resolving the secondary qualities of light, sound, etc., into modes of motion, while representing the object very differently from the unscientific mind, he agrees with this in holding to the reality of something external, regarding this as antecedent to and therefore as independent of the particular mind which receives the sense-impression.  Again, he assumes the uniformity of nature, the universality of the causal relation, and so on.