DESIGN FOR A BRAIN 3/14 



organ in the body, as a specialised means to survival (S. 1/10). 

 We shall use the concept of ' survival ' repeatedly; but before we 

 can use it, we must, by S. 2/10, transform it to our standard 

 form and say what it means 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. 

 Further, there can be no doubt about the objectivity of the 

 difference, for they fetch quite different prices in the market. 

 The distinction must be capable of objective definition. 



It is suggested that the definition may be obtained in the 

 following way. That an animal should remain ' alive ', certain 

 variables must remain without certain ' physiological ' limits. 

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

 the species is fixed. In practice one does not experiment on 

 animals in general, one experiments on one of a particular species. 

 In each species the many physiological 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 

 related 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 methods of 

 Chapter 2 ? There is no difficulty. Given a species, we observe 

 what follows when members of the species are started from a 

 variety of initial states. We shall find that large initial changes 

 in some variables are followed in the system by merely transient 

 deviations, while large initial changes in others are followed by 

 deviations that become ever greater till the ' machine ' changes 



42 



