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. The pretended rational doctrine of the substantiality and spirituality of the soul, with all the apparatus that accompanies it, is born simply of the necessity which men feel of grounding upon reason their inexpugnable longing for immortality and the subsequent belief in it. All the sophistries which aim at proving that the soul is substance, simple and incorruptible, proceed from this source. And further, the very concept of substance, as it was fixed and defined by scholasticism, a concept which does not bear criticism, is a theological concept, designed expressly to sustain faith in the immortality of the soul.