aid 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.

Similarly, the modern psychologist, when confining himself within the
limits of positive science, and treating mind phenomenally or
empirically, or, in other words, tracing the order of mental states in
time and assigning their conditions, takes for granted much the same as
physical science does. Thus, as our foregoing analysis of perception
shows, he assumes that there is an external cause of our sensations,
that the