Certainly one thing which stands in the center of all mental life seems entirely incomprehensible as the result of a mere mechanics of ideas, that is, that unity of mind without which we could not speak of personality, of character, of individuality, without which we could not call one man haughty and another humble, one good and another bad, one noble and another base. Because of this weakness in the theory numerous great thinkers, Rousseau, Kant, Fichte, Schopenhauer, raised their voices to insist upon the significance of the life of feeling and will as well as of the life of ideas, even to give to the former the first place, as the expression of mind’s most real inner being. Thus intellectualism was opposed by what we now call voluntarism. This transferring of the conceptions of natural science to psychological research, in spite of the mighty impulse it gave to psychology, was not without its disadvantage. The first brilliant advances in natural science were in the province of physics, especially of mechanics. It is no wonder, then, that psychologists, in their gropings after something similar, turned first to mechanical-physical processes. Inertia, attraction, and repulsion, as we have seen, aggregation and chemical combination, were the categories with which they worked. No wonder, either, that facts were often distorted and their comprehension made difficult. For if mind is a machine, it is certainly not such a machine as even the most ingeniously constructed clock or as a galvanic battery. It is bound up with the organic body, especially with the nervous system, and on the structure and functions of the nervous system its own existence and activity somehow depend. So, if one wishes to use material analogies and to make them fruitful for the comprehension of mental structures, they must be taken from organic life, from biology rather than from physics and chemistry. We may find phenomena comparable to individuality and character, to the mind’s feeling and willing, in the unitary existence of every plant and animal organism, in the peculiar determination of its instinct of life and in the many special branches into which this instinct ceaselessly unfolds. And indeed the specifically mechanical categories gradually disappeared from psychology during the nineteenth century, and made way for the biological categories--reflex, inhibition, practice, assimilation, adaptation, and so on. Especially that great acquisition of modern biology, the theory of evolution, was at once seized upon by psychologists, and was utilized for gaining an understanding of the processes as well in the mind of the individual as in human society.