t scientific realm. The
astronomer cannot find by observation that there is no star the
movements of which are not the effects of foregoing causes. He knows it
beforehand, he demands it, he does not recognize any movement as
understood until he has found the causes, he presupposes that such
causes exist, that no star moves simply by a magic power, and that
nowhere in the astronomical universe is the chain of causality broken.
He postulates it, and where he does not discover the causes, he is sure
that he has not solved the real problem.

In the same way the psychologist who aims towards explanation of mental
facts must postulate that there cannot be any mental state which is not
an accompaniment of a physical brain process, and is as such connected
through physical means with the preceding and the following events in
the psychophysical system. Only when such a general framework of theory
is built up by a logical postulate, is the way open to make use of all
those observations of the laboratory and of the clinic, of the zooelogist
and of the anatomist. It is the theory which has to give the right
setting to those scattered observations. However far we may be from
being able to point to the special brain process which lies at the
bottom of the higher mental state, we know beforehand that there is no
shadow of an idea, no fringe of a feeling, no suggestion of a desire
which does not correspond to definite processes in the brain. The
details may and must be material for diverging theories, but the
conflict of such hypothetical opinions has nothing to do with the
certainty of the underlying conviction that if we knew the whole truth,
we should recognize every single mental happening as parallel to
physical processes in the nervous system. To explain mental facts means
to think them as parallel to the brain processes which have their own
causal connections in the physical world.

We started, for instance, from the old observation that two impressions
which come to our mind at the same time have a tendency to reawaken one
another; and we saw that psychology was well able to formulate these
facts in general statements of the association of ideas. But we realized
that that in itself is not really explanation. If the odor which we
smell awakes in us the name of a chemical substance, and if we now bring
this under the general heading of association of ideas, an explanation
is not really given by it. That smell sensation itself is