of the
psychological experiment the mental variations which occur with changes
of physical conditions. We might feel, without instruments, that our
ideas pass on more easily after a few cups of strong coffee, but the
laboratory may measure that with its exact methods and study in
thousandth parts of a second, the quickening or retarding in the flow of
ideas. Every subjective illusion is then excluded, our electrical
clocks, which measure the rapidity of mental action and of thought
association, will show then beyond doubt how every change in the
organism influences the processes of the mind. Bodily fatigue and
indigestion, physical health and blood circulation, everything,
influence our mental make-up. In the same way it is the laboratory
experiment which shows by the subtlest means that every mental state
produces bodily effects where we ordinarily ignore them. As soon as we
apply the equipment of the psychological workshop, it is easy to show
that even the slightest feeling may have its influence on the pulse and
the respiration, on the blood circulation and on the glands; or, that
our thoughts give impulse to our muscles and move our organs when we
ourselves are entirely unaware of it.

Again we may turn in another direction. Pathology shows us how every
physical disablement of the brain is accompanied by mental processes.
If the blood supply to the brain is cut off, we faint; a blow on the
head may wipe out the memory of the preceding hours, and a hemorrhage in
the brain, the bursting of a blood vessel which destroys groups of brain
cells, produces serious defects in the mental content. A tumor in the
brain may completely change the personality; the bodily disease of
certain convolutions in the brain brings with it the loss of the power
of speech; paralysis of the brain dissolves the whole mental
personality. Physical inhibition in the growth of the brain involves, on
the mental side, feeble-mindedness and idiocy. Of course, all this is
not sufficient to bring out a definite parallelism between special
mental functions and special physical processes, as the phenomena are
extremely complex. If a patient who has suffered from a mental
disturbance dies, and his brain is examined, there is no simple
correlation before us. It may be difficult to diagnose exactly the
mental symptoms. If we have heard that the man was unable to read, we do
not know from that what really happened in his brain. He may not have
read because