The one traces the development of complex organs from simple rudiments; the other follows the building up of complex conceptions out of simpler constituents of thought.

As the physiologist inquires into the way in which the so-called "functions" of the body are performed, so the psychologist studies the so-called "faculties" of the mind.

Even a cursory attention to the ways and works of the lower animals suggests a comparative anatomy and physiology of the mind; and the doctrine of evolution presses for application as much in the one field as in the other.
But there is more than a parallel, there is a close and intimate connexion between psychology and physiology.

No one doubts that, at any rate, some mental states are dependent for their existence on the performance of the functions of particular bodily organs.

There is no seeing without eyes, and no hearing without ears.