HAECKEL'S PHILOSOPHY. 409 



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 supple- 

 ments the insufficient combining power of the understanding and by 

 association of memory-images combines remote cognitions into a con- 

 nected whole. In this way new ideas arise which alone explain the per- 

 ceived 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 sur- 

 mise 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, tpo, 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 assum- 

 ing 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 rea- 

 son.* 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. 



