The problem which first presents itself to us is that of Life in general. The problem is inseparable from that of corporeality. If we follow phenomena to their last conceivable reduction, we finally pass from the perception of mass to the concept of matter; but further than this we cannot go. At least, we can perceive only material things, and that which we call the spiritual in distinction from the corporeal has always something corporeal as its basis; and if we do not wish to dispense with the reliable guidance of experience, we shall not overleap this barrier. Science cannot reckon with supernatural factors. What matter _is_ we cannot know; that it exists, however, that the phenomena of nature are no empty seeming, sensation, as the felt result of the mutual relation between us and the outer world, testifies. Sensation is the basis of our self-consciousness, of the only full and irrefutable certainty that we possess. As to what true Being or Existence is, there is disagreement; but there can be none regarding the fact that we are conscious of our sensations; and upon this consciousness rests the postulate of the materiality[66] of all existence. In order to assert the materiality of all phenomena, we are forced to distinguish between a corporeal and a non-corporeal action of matter; matter operates mentally when its division or differentiation proceeds so far that the resulting phenomena can no longer be perceived by the senses, but only conceived by thought. The indivisibility of mind from corporeality follows directly from this definition of the mental side of nature. We distinguish between the two only for convenience' sake. The newer Psychology knows nothing of Sensuality in the old sense of the word, since the basis of all psychical effects is physical. For matter operating mentally, as for matter operating corporeally, there are no specific energies; it is, as Wundt expresses it, functionally indifferent. The differing results of a high differentiation of centralized organisms arise in accordance with the changing combinations of elementary parts and nerve activities. These results are not, however, to be regarded as the mere effects of matter, but as phenomena of the same, in fact, as the consummation and crown of the whole evolution of nature. Even in the sense-organs we see the differentiation of matter advance beyond the sphere of sense-perception. Therefore, in distinguishing between mind and matter, we are still in the realm of the natural, and follow the path of experience, if by experience is understood not alone immediate experience, but also the conclusions which directly or by strict analogy may be drawn from it.