Kant, whose criticism found its point of departure in Hume, attempted to establish the rationality of this longing for immortality and the belief that it imports; and this is the real origin, the inward origin, of his Critique of Practical Reason, and of his categorical imperative and of his God.  But in spite of all this, the sceptical affirmation of Hume holds good.  There is no way of proving the immortality of the soul rationally.  There are, on the other hand, ways of proving rationally its mortality.

It would be not merely superfluous but ridiculous to enlarge here upon the extent to which the individual human consciousness is dependent upon the physical organism, pointing out how it comes to birth by slow degrees according as the brain receives impressions from the outside world, how it is temporarily suspended during sleep, swoons, and other accidents, and how everything leads us to the rational conjecture that death carries with it the loss of consciousness.  And just as before our birth we were not, nor have we any personal pre-natal memory, so after our death we shall cease to be.  This is the rational position.

The designation “soul” is merely a term used to denote the individual consciousness in its integrity and continuity; and that this soul undergoes change, that in like manner as it is integrated so it is disintegrated, is a thing very evident.  For Aristotle it was the substantial form of the body—­the entelechy, but not a substance.  And more than one modern has called it an epiphenomenon—­an absurd term.  The appellation phenomenon suffices.

Rationalism—­and by rationalism I mean the doctrine that abides solely by reason, by objective truth—­is necessarily materialist.  And let not idealists be scandalized thereby.

The truth is—­it is necessary to be perfectly explicit in this matter—­that what we call materialism means for us nothing else but the doctrine which denies the immortality of the individual soul, the persistence of personal consciousness after death.

In another sense it may be said that, as we know what matter is no more than we know what spirit is, and as matter is for us merely an idea, materialism is idealism.  In fact, and as regards our problem—­the most vital, the only really vital problem—­it is all the same to say that everything is matter as to say that everything is idea, or that everything is energy, or whatever you please.  Every monist system will always seem to us materialist.  The immortality of the soul is saved only by the dualist systems—­those which teach that human consciousness is something substantially distinct and different from the other manifestations of phenomena.  And reason is naturally monist.  For it is the function of reason to understand and explain the universe, and in order to understand and explain it, it is in no way necessary for the soul to be an imperishable substance.  For the purpose of explaining and understanding our psychic life, for psychology, the hypothesis of the soul is unnecessary.  What was formerly called rational psychology, in opposition to empirical psychology, is not psychology but metaphysics, and very muddy metaphysics; neither is it rational, but profoundly irrational, or rather contra-rational.