DESIGN FOR A BRAIN 



2/5 



Table 2/4/1 



variables or omitted old. These new combinations were new 

 systems. 



2/5. Because any real ' machine ' has an infinity of variables, 

 from which different observers (with different aims) may reason- 

 ably make an infinity of different selections, there must first be 

 given an observer (or experimenter); a system is then defined as 

 any set of variables that he selects from those available on the 

 real * machine '. It is thus a list, nominated by the observer, 

 and is quite different in nature from the real ' machine '. Through- 

 out the book, ' the system ' will always refer to this abstraction, 

 not to the real material i machine '. 



Among the variables recorded will almost always be ' time ', so 

 one might think that this variable should be included in the list 

 that specifies the system. Nevertheless, time comes into the 

 theory in a way fundamentally different from that of all the 

 others. (The difference is shown most clearly in the canonical 

 equations of S. 19/9.) Experience has shown that a more con- 

 venient classification is to let the set of variables be divided into 

 4 system ' and 4 time '. Time is thus not to be included in the 

 variables of the system. In Table 2/4/1 for instance, ' the 

 system ' is defined to be the three variables on the right. 



2/6. The state of a system at a given instant is the set of numerical 

 values which its variables have at that instant. 



Thus, the six-variable system of S. 2/3 might at some instant 

 have the state: —4°, 0-3 radians/sec, 128°, 52 cm., 42-8 minutes, 

 88-4 cm. 



Two states are equal if and only if the two numerical values in 

 each pair are equal, all pairs showing equality. 



16 



