22 IRRITABILITY 



every happening or state, namely, the cause and the conditions, 

 leads to new difficulties, for then, upon a more exact analysis 

 arises the question: Which is the cause and what are the condi- 

 tions ? It is very soon found, however, that this does not permit 

 of any strict differentiation, as the two conceptions can not be 

 sharply separated. This difficulty was brought to my notice with 

 particular force during an animated discussion with a friend 

 and colleague about twenty years ago, which I have always 

 remembered. I had observed at that time the dependence of 

 pseudopod formation of amoeboid cells on the oxygen of the 

 medium, and had found that the expansion phase of proto- 

 plasmic movement, that is, the extension of pseudopods, the 

 centrifugal flowing of the protoplasm into the surrounding 

 medium and with this the enlargement of the surface of the cell 

 body, only takes place when oxygen is contained in the sur- 

 rounding medium and never occurs in its absence. Being at that 

 time wholly under the influence of the conception of cause, I 

 believed that oxygen was the cause of the formation of the 

 pseudopods. To this my friend made the objection: "Yes, I 

 quite acknowledge the fact of the dependence of the formation 

 of pseudopods on oxygen, but what informs me that the oxygen 

 is really the cause? It might be simply a necessary condition." 

 This objection led to a long debate, which ended, however, with- 

 out our being able to agree. We were not in a position to dis- 

 tinguish between the conception of cause and that of condition, 

 and at that time the idea did not occur to us to emancipate 

 ourselves from the conception of cause deeply implanted in us 

 as a result of our training. In fact, one is greatly embarrassed 

 if one attempts to sharply distinguish by a definition the concep- 

 tion of cause and that of condition. A condition is a factor on 

 which a state or a process is dependent for its existence or its 

 taking place. To the conception of condition belongs, besides 

 the factor of relation, that of necessity. Every condition is neces- 

 sary to the existence or taking place of this state or process. 

 Without the condition in question the state or process does not 

 occur. The same must be demanded for the conception of cause. 

 No state exists, no process takes place, without its cause. The 



