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 there 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 is anything real independently of the mind, and if so, what.  In doing this it inquires not only what common sense