THE ANIMAL AS MACHINE 3/14 



the more general. For this reason the environment will always 

 be treated as wholly dynamic. 



Essential variables 



3/14. The biologist must view the brain, not as being the seat of 

 the ' mind ', nor as something that ' thinks ', but, like every other 

 organ in the body, as a specialised means to survival. We shall 

 use the concept of ' survival ' repeatedly ; but before we can use 

 it, we must, by S. 2/8, transform it to our standard form. What 

 does it mean in terms of primary operations ? 



Physico-chemical systems may undergo the most extensive 

 transformations without showing any change obviously equivalent 

 to death, for matter and energy are indestructible. Yet the dis- 

 tinction between a live horse and a dead one is obvious enough 

 — they fetch quite different prices in the market. The distinc- 

 tion must be capable of objective definition. 



It is suggested that the definition may be obtained in the fol- 

 lowing way. That an animal should remain ' alive ' certain 

 variables must remain within certain ' physiological ' limits. 

 What these variables are, and what the limits, are fixed when 

 we have named the species we are working with. In practice 

 one does not experiment on animals in general, one experiments 

 on one of a particular species. In each species the many physio- 

 logical variables differ widely in their relevance to survival. 

 Thus, if a man's hair is shortened from 4 inches to 1 inch, the 

 change is trivial ; if his systolic blood-pressure drops from 120 mm. 

 of mercury to 30, the change will quickly be fatal. 



Every species has a number of variables which are closely re- 

 lated to survival and which are closely linked dynamically so 

 that marked changes in any one leads sooner or later to marked 

 changes in the others. Thus, if we find in a rat that the pulse- 

 rate has dropped to zero, we can predict that the respiration 

 rate will soon become zero, that the body temperature will soon 

 fall to room temperature, and that the number of bacteria in the 

 tissues will soon rise from almost zero to a very high number. 

 These important and closely linked variables will be referred to as 

 the essential variables of the animal. 



How are we to discover them, considering that we may not use 

 borrowed knowledge but must find them by the method of 



41 



