3/11 THE ORGANISM AS MACHINE 



will pass to the nervous system, and will be switched through it, 

 if he is a trained rider, by such a route that they, or the effects 

 set up by them, will excite to activity those muscles which push 

 the right hand forwards. 



We can now specify the variables which must compose the 

 system if it is to be state-determined. We must include: the 

 angular position of the handlebar, the velocity of lateral movement 

 of the two points of contact between wheels and road, the distance 

 laterally between the line joining these points and the point 

 vertically below the rider's centre of gravity, and the angular 

 deviation of the rider from the vertical. These four variables are 

 defined by S. 3/8 to be the ' environment ' of the rider. (Whether 

 the fourth variable is allotted to ' rider ' or to ' environment ' is 

 optional (S. 3/12). To make the system state-determined, there 

 must be added the variables of the nervous system, of the relevant 

 muscles, and of the bone and joint positions. 



As a second example, consider a butterfly and a bird in the air, 

 the bird chasing the butterfly, and the butterfly evading the bird. 

 Both use the air around them. Every movement of the bird 

 stimulates the butterfly's eyes and this stimulation, acting through 

 the butterfly's nervous system, will cause changes in the butter- 

 fly's wing movements. These movements act on the enveloping 

 air and cause changes in the butterfly's position. A change of 

 position immediately changes the excitations in the bird's eye, 

 and this leads through its nervous system to changed movements 

 of the bird's wings. These act on the air and change the bird's 

 position. So the processes go on. The bird has as environment 

 the air and the butterfly, while the butterfly has the air and the bird. 

 The whole may reasonably be assumed to be state-determined. 



3/11. The organism affects the environment, and the environ- 

 ment affects the organism : such a system is said to have i feed- 

 back ' (S. 4/ 14 )- 



The examples of the previous section provide illustration. The 

 muscles in the rider's arm move the handlebars, causing changes 

 in the environment; and changes in these variables will, through 

 the rider's sensory receptors, cause changes in his brain and 

 muscles. When bird and butterfly manoeuvre in the air, each 

 manoeuvre of one causes reactive changes to occur in the other. 



The same feature is shown by the example of S. 1/17 — the 



37 



