the anatomical equipment by which they become ready for
functioning. Most of the tracts at first lack the so-called medullar
sheath, and from month to month new paths are provided with this
physical equipment.

Finally we have the experiment of the physiologist. His vivisectional
experiments, for instance, demonstrate that the electrical stimulation
of a definite spot on the surface of a dog's brain produces movements
which we should ordinarily take as expressions of mental states,
movements of the front legs or of the tail, movements of barking or
whining. On the other hand, the dog becomes unable to fulfill the mental
impulses if certain definite parts of his brain are destroyed. The
physiologist may show from the monkey down to the pigeon, to the frog,
to the ant, to the worm, how the behavior of animals is changed as soon
as certain groups of nervous elements are extirpated. It is the mental
emotional character of the pigeon which is changed when the physiologist
cuts off parts of his brain. In short, stimulation and destruction
demonstrate, by experiments which supplement each other, that mental
functions correspond to brain functions.

There is thus no lack of demonstration from all quarters that mental
facts and brain processes belong together; and yet, however much we may
cumulate such popular and scientific observations, they would never by
themselves admit of the sweeping generalization that there cannot be any
mental state which is not accompanied by a process in the central
nervous system. Someone might say, to be sure, the perceptions and
memory images, the volitions and instincts and impulses, have their
physiological basis, but there remain after all acts of attention, or
decisions, or subtle feelings, or flights of imagination, which are
independent of any brain action. Here, indeed, observation cannot settle
such a general principle. Its real hold lies in the fact with which we
started: there is no causal connection in the mental states as such. If
we want to understand mental facts as such in a chain, of causal events,
we have first to conceive them as parallel to physical events. The
principle of psychophysical parallelism, that is, the principle that
every psychical process accompanies a physiological change is thus not a
mere result of observation. It is simply a postulate. Every science
begins with postulates and only that which fulfills such postulates has
the dignity of truth in the midst of tha