re are material bodies in space, which act on our sense-organs
and so serve as the condition of our sense-impressions. More than this,
he regards, in the way that has been illustrated in this work, the
percept itself, in so far as it is a process in time, as the normal
result of the action of such external agents on our nerve-structures, in
other words, as the effect of such action in the case of the healthy and
perfect nervous organism with the average organized dispositions,
physical and psychical; in which case he supposes the percept to
correspond, in certain respects at least, with the external cause as
made known by physical science. And, on the other hand, he looks on a
false or illusory percept as arising in another way not involving, as
its condition, the pre-existence of a corresponding material body or
physical agent. And in this view of perception, as of other mental
phenomena, the psychologist clearly takes for granted the principle that
all mental events conform to the law of causation. Further, he assumes
that the individual mind is somehow, in a way which it is not his
province to inquire into, one and the same throughout, and so on.

The doctrine of evolution, too, in so far as scientific--that is, aiming
at giving an account of the historical and pre-historical developments
of the collective mind in time--agrees with psychology in making like
assumptions. Thus, it conceives an external agency (the environment) as
the cause of our common sensations and perceptions. That is to say, it
represents the external world as somehow antecedent to, and so
apparently independent of, the perceptions which are adjusted to it. And
all this shows that science, while removed from vulgar unenlightened
opinion, takes sides with popular thought in assuming the truth of
certain fundamental ideas or so-called intuitive beliefs, into the exact
meaning of which it does not inquire.

When the meaning of these assumptions is investigated, we pass out of
the scientific into the philosophic domain. Philosophy has to critically
investigate the data of popular thought and of science. It has to
discover exactly what is implied in these fundamental principles. Then
it has to test their value by erecting a final criterion of truth, by
probing the structure of cognition to the bottom, and determining the
proper organ of certain or accurate knowledge; or, to put it another
way, it has to examine what is meant by reality, whether there i