2/3 DESIGN FOR A BRAIN 



must have the peculiarity that it is applicable to all ; it must, 

 so to speak, specialise in generality. 



Variable and system 



2/3. The first step is to record the behaviours of the machine's 

 individual parts. To do this we identify any number of suitable 

 variables. A variable is a measurable quantity which at every 

 instant has a definite numerical value. A ' grandfather ' clock, 

 for instance, might provide the following variables : — the angular 

 deviation of the pendulum from the vertical ; the angular velocity 

 with which the pendulum is moving ; the angular position of a 

 particular cog-wheel ; the height of a driving weight ; the 

 reading of the minute-hand on the scale ; and the length of the 

 pendulum. If there is any doubt whether a particular quantity 

 may be admitted as a 4 variable ' I shall use the criterion whether 

 it can be represented by a pointer on a dial. I shall, in fact, 

 assume that such representation is always used : that the 

 experimenter is observing not the parts of the real ' machine ' 

 directly but the dials on which the variables are displayed, as 

 an engineer watches a control panel. 



Only in this way can we be sure of what sources of information 

 are used by the experimenter. Ordinarily, when an experimenter 

 examines a machine he makes full use of knowledge ' borrowed ' 

 from past experience. If he sees two cogs enmeshed he knows 

 that their two rotations will not be independent, even though he 

 does not actually see them rotate. This knowledge comes from 

 previous experiences in which the mutual relations of similar 

 pairs have been tested and observed directly. Such borrowed 

 knowledge is, of course, extremely useful, and every skilled 

 experimenter brings a great store of it to every experiment. 

 Nevertheless, it must be excluded from any fundamental method, 

 if only because it is not sufficiently reliable : the unexpected 

 sometimes happens ; and the only way to be certain of the rela- 

 tion between two parts in a new ' machine ' is to test the rela- 

 tionship directly. 



All the quantities used in physics, chemistry, biology, physio- 

 logy, and objective psychology, are variables in the defined sense. 

 Thus, the position of a limb can be specified numerically by co- 

 ordinates of position, and movement of the limb can move a pointer 



14 



