The soul, in fine, is the formal principle or primary source of the threefold life in man, namely, the metabolic life, which man shares with plants, the sentient life, which he shares with animals, and the rational life, which is uniquely human. The human soul is often spoken of as the mind. In their dictionary sense, both terms denote one and the same reality, namely, the human entelechy or vital principle in man, but the connotation of these terms is different. The term soul signifies the vital principle in so far as it is the primary source of every kind of life in man, that is, vegetative, sentient, and rational. The term mind, however, connoting conscious rather than unconscious life, signifies the vital principle in so far as it is the root and ground of our conscious life (both sentient and rational). Here, however, the distinction is of no great moment, and the terms may be regarded as synonymous. The definitions which we have given are, of course, blasphemous in the ears of our modern neo-Kantian phenomenalists, whose preference is for a _functional_, rather than a _substantial_, mind or soul; but we will pay our respects to them later. It is clear, however, from what has been said, that, for evidences of the superiority and spirituality of the human soul, we must recur, not to the external manifestations of our nutritive life, but to the internal manifestations of our conscious life. The latter are wholly inaccessible to the external senses and perceptible only to the intuition of consciousness, introspection, or internal experience, as it is variously called. All our self-knowledge rests on the basis of introspection, and without it the science of psychology would be impossible. In fact, not only psychology, but the physical sciences as well, depend for their validity on the testimony of consciousness; for the external world is only knowable to the extent that it enters the domain of our consciousness. Recently, as we have seen, a tendency to discredit internal experience has arisen among materialistic extremists. This “tendency,” to quote the words of Keyser, “most notably represented by the behaviorist school of psychologists (like Professor Watson, for example), is manifest in the distrust of introspections as a means of knowledge of mental phenomena and in the growing dependence of psychology upon external observation of animal and human behavior and upon physiological experiment, as if matter were regarded ‘as something much more solid and indubitable than mind’ (Bertrand Russell).”—C. J. Keyser, _Science_, Nov. 25, 1921, p. 520. Since, however, all our knowledge depends on the validity of consciousness, such a tendency is suicidal and destructive of all science, whether physical or psychological. The attempts, therefore, of mechanists, like Loeb, and behaviorists, like Watson, to dispense with consciousness overreach themselves. For how can the mechanists _know_