3/3 DESIGN FOR A BRAIN 



3/3. Few would deny that the elementary physico-chemical 

 events in the living organism can be treated as variables. But 

 some may hesitate before accepting that readings on dials are 

 adequate for the description of all significant biological events. 

 As the remainder of the book will assume that they are sufficient, 

 I must show how the various complexities of biological experience 

 can be reduced to this standard form. 



A simple case which may be mentioned first occurs when an 

 event is recorded in the form ' strychnine was injected at this 

 moment ', or ' a light was switched on ', or ' an electric shock was 

 administered '. Such a statement treats only the positive event 

 as having existence and ignores the other state as a nullity. It 

 can readily be converted to a numerical form suitable for our 

 purpose by using the device mentioned in S. 2/3. Such events 

 would then be recorded by assuming, in the first case, that the 

 animal always had strychnine in its tissues but that at first the 

 quantity present was mg. per g. tissue ; in the second case, that 

 the light was always on, but that at first it shone with a brightness 

 of candlepower ; and in the last case, that an electric potential 

 was applied throughout but that at first it had a value of volts. 

 Such a method of description cannot be wrong in these cases for 

 it defines exactly the same set of objective facts. Its advantage 

 from our point of view is that it provides a method which can be 

 used uniformly over a wide range of phenomena : the variable is 

 always present, merely varying in value. 



But this device does not remove all difficulties. It sometimes 

 happens in physiology and psychology that a variable seems to have 

 no numerical counter-part. Thus in one experiment two cards, 

 one black and one brown, were shown alternately to an animal as 

 stimuli. One variable would thus be 4 colour ' and it would have 

 two values. The simplest way to specify colour numerically is to 

 give the wave-length of its light ; but this method cannot be used 

 here, for ' black ' means ' no light ', and ' brown ' does not occur 

 in the spectrum. Another example would occur if an electric 

 heater were regularly used and if its switch indicated only the 

 degrees 4 high ', ' medium ', and i low '. Another example is given 

 on many types of electric apparatus by a pilot light which, as a 

 variable, takes only the two values ' lit ' and 4 unlit '. More 

 complex examples occur frequently in psychological experiments. 

 Table 2/5/1, for instance, contains a variable ' part of skin stimu- 



30 



