108 ORGANISM AND MECHANISM 



concept of energy is emphasised, or a chemico-physical de- 

 scription which is ideally mechanical, that is, theoretically 

 reducible to matter-and-motion description, though, as a mat- 

 ter of fact, the reduction may not have been as yet effected. 

 A mechanistic description, in short, is in terms of the funda- 

 mental concepts of physics and chemistry; and it is the 

 most precise and most thorough kind of description that is 

 known. 



= Given three good observations of a comet, an astronomer 

 who knows his business can prophesy with certainty when, 

 barring accidents, it will return. He may not tell us what 

 gravitation means, or what the comet is made of, or how 

 it arose, or what it portends to mankind, but of the com- 

 ing and going he gives a complete account, as the punctual 

 return of the comet afterwards proves. Now the question 

 which interests us at present is not whether the biologist, 

 if he knew his business as well as the astronomer, could 

 tell us at what precise date next spring the swallows will 

 reach our shores, but rather whether the success of his predic- 

 tion depends on the reduction of the swallows' behaviour 

 to mechanistic formulation. 



The question may be split into two. The first is: How 

 far, as a matter of fact, can characteristically vital occur- 

 rences, such as the contraction of a muscle, be described in 

 terms of the formula which serve for the study of tides 

 and eclipses, the moulding of a dew-drop or the making of 

 a star ? One obvious limit is that, if the organism has mental- 

 ity that counts in its agency, then the behaviour cannot be 

 completely formulated in mechanical terms. Mind cannot 

 be described in terms of matter, or emotion in terms of mo- 

 tion. As there are some biologists, such as Prof. Jacques 

 Loeb, who hold " a tropism theory of animal conduct " which 



