METHODS OF PHYSIOLOGICAL RESEARCH 5 



appears to him living and which lifeless, would invariably pronounce 

 the yeast-cells and the fat-droplets lifeless, the dancing granules 

 living ; but the latter are nothing more than minute calcareous 

 ■crystals, so light that they are put into trembling motion passively 

 by the excessively delicate motion that the particles of every liquid 

 possess. The manifestation of motion, which, because we see no 

 external source, we are inclined to ascribe to an internal cause, 

 here misleads to the assumption of life. Such examples may be 

 found in unlimited number. 



Hence, under certain circumstances it is not at all easy to dis- 

 tinguish the living from the lifeless, and it is accordingly clear 

 that the first duty of physiology must be to inquire after the 

 ■criteria of such a distinction, i.e., mentally to circumscribe the 

 subject-matter, life, in relation to non-living nature. 



Not less great are the difficulties that we meet when we 

 ■consider the second idea that is included in the task of physio- 

 logy, that of investigation. What is meant by investigation or 

 ■explanation ? 



Civilised man appears to be distinguished essentially from 

 primitive races by a great desire, namely, that of seeking after the 

 ■causes of phenomena, or, in other words, a craving for causalitj'. 

 This longing in all things to ask " why," from a pure desire for 

 knowledge apart from any practical aim, appears to be an acquisi- 

 tion of civilisation, and its origin and development can be seen 

 clearly in children of a certain age. When we have discovered a 

 cause for any phenomenon, the craving for causality in that respect 

 is satisfied ; we have investigated and explained the phenomenon. 

 This is true of investigation in all departments of science, of 

 historical and philological science as well as that of nature, in so 

 far as the development of the science has progressed beyond the 

 stage characterised by the mere accumulation of facts. But when 

 we have discovered the immediate cause of any phenomenon, we 

 have satisfied the craving for causality only relatively, for the cause 

 itself is a phenomenon that must be explained. Thus gradually 

 .and systematically we put individual phenomena and series of 

 phenomena into causal connection with one another, and constantly 

 reduce larger and larger groups to their causes. Ultimately, how- 

 ever, the question arises how far this reduction may be carried 

 .successfully. Is there a final cause for the phenomena, or may 

 the reduction be continued to infinity ? 



In all fields of non-living nature, especially in physics and 

 •chemistry, investigation has shown that all phenomena thus far 

 known and investigated may be reduced in the last instance to a 

 single common cause, namely, the movement of very small material 

 elements. The whole physical world is conceived as consisting of 

 separate, indivisible, extremely small, elementary particles called 



