36 The Universe and Life 



taken various forms. One line of argument is of a 

 strictly a priori character, not based upon experi- 

 ence or experimentation. There can be no functional 

 relations, no relations of cause and effect, it urges, 

 between things that are so totally different in kind as 

 are motions and emotions, as are particles and sensa- 

 tions, as are actions and thought. A wish, an idea, 

 cannot produce or change a motion, it is held, because 

 a wish or an idea is in its very essence a different kind 

 of a thing from a motion. And for the same reason, a 

 motion, a rise in temperature, any physical change, 

 cannot alter sensations, ideas, wishes. 



But such an argument mistakes the nature of scien- 

 tific knowledge. Science is man's attempt to organize 

 his experiences, to formulate observed relations. It is 

 compelled to deal with many things that are ulti- 

 mate; things that are to be learned only by experi- 

 encing them. Its final foundation is critical observa- 

 tion and experimentation. To these it must be loyal 

 to the end, if it is indeed to present itself as a formu- 

 lation of experience. If it abandons them, it is lost ; it 

 then becomes merely one more system of metaphysics. 



And critical observation and experimentation show 

 that mental states and physical conditions are not 

 independent. Heat and cold yield different sensa- 

 tions. Cutting the flesh with a knife yields very dif- 

 ferent sensations, emotions, and purposes from those 

 that occur when this physical change does not take 

 place. The physical objects that are present in a 

 room determine what we see, determine what sensa- 

 tions our eyes receive. No matter how refined the 



