But our sense-activity is limited; we can discover only a part of the qualities possessed by the objects of the external world. The civilized man transforms his sense-impressions into specific sensations in the sense-centers of the cortex, and combines these by association in the thought-centers into ideas or presentations, and by further combination of the idea-groups he finally reaches connected knowledge. But this knowledge always remains unsatisfactory, unless the fancy supplements the insufficient combining power of the understanding and by association of memory-images combines remote cognitions into a connected whole. In this way new ideas arise which alone explain the perceived facts and satisfy the causal need of the reason.
The ideas which fill the gaps in our knowledge, or take its place, we may call belief (Glaube). We are forced to belief in science. We surmise or assume that a certain relation exists between two phenomena although we do not know it with certainty. In the case of knowledge of causes we form a hypothesis. But only such hypotheses can be admitted in science which do not contradict known facts, e. g., in physics, the doctrine of ether-vibrations; in chemistry, the assumption of atoms and their affinity; in biology, the doctrine of the molecular structure of the living plasma.
The explanation of a larger series of connected phenomena by the assumption of a common cause we call a theory. Here, too, faith or belief in the scientific sense is indispensable, here, too, the poetic fancy fills the gap which the understanding leaves in the knowledge of things. A theory can therefore be regarded only as an approximation to truth; it can always be supplanted by a better theory. But theory is indispensable in science, for theory alone explains the facts by assuming causes. Hence whoever wishes to do entirely without theory and to construct pure science upon nothing but 'certain facts' relinquishes all knowledge of causes and the satisfaction of the causal need of the reason. Examples of such theories are the theory of gravitation, the cosmological nebular theory, the principle of energy, the atomic theory, the vibration theory, the cell theory, the theory of descent. They explain a system of natural phenomena by assuming a common cause for all the particular facts of their territory. This cause itself may be unknown in its essence or be a merely provisional hypothesis. Gravitation, energy itself, ether, the atom, heredity, may be regarded by sceptical philosophers as 'mere hypotheses,' as products of scientific belief, but they are indispensable until they are replaced by a better hypothesis.
This theory of knowledge is almost identical with that offered by Epicurus three centuries before Christ. With respect to the problem
- See also 'Monismus,' p. 37.