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types. At one extreme is Newton's theory of gravitation — at once 

 simple, and precise, and exactly true. When such a combination 

 is possible, Science is indeed lucky ! Darwin's theory, on the 

 other hand, is not so simple, is of quite low accuracy numerically, 

 and is true only in a partial sense — that the simple arguments 

 usually used to apply it in practice (e.g. how spraying with D.D.T, 

 will ultimately affect the genetic constitution of the field mouse, 

 by altering its food supply) are gross simplifications of the complex 

 of events that will actually occur. 



The theory attempted in this book is of the latter type. The 

 real facts of the brain are so complex and varied that no theory 

 can hope to achieve the simplicity and precision of Newton's; 

 what then must it do ? I suggest that it must try to be exact in 

 certain selected cases, these cases being selected because there we 

 can be exact. With these exact cases known, we can then face 

 the multitudinous cases that do not quite correspond, using the 

 rule that if we are satisfied that there is some continuity in the 

 systems' properties, then insofar as each is near some exact case, 

 so will its properties be near to those shown by the exact case. 



This scientific strategy is by no means as inferior as it may 

 sound; in fact it is used widely in many sciences of good repute. 

 Thus the perfect gas, the massless spring, the completely reflecting 

 mirror, the leakless condenser are all used freely in the theories 

 of physics. These idealised cases have no real existence, but they 

 are none the less important because they are both simple and 

 exact, and are therefore key points in the general theoretical 

 structure. 



In the same spirit this book will attend closely to certain 

 idealised cases, important because they can be exactly defined and 

 because they are manageably simple. Maybe it will be found 

 eventually that not a single mechanism in the brain corresponds 

 exactly to the types described here ; nevertheless the work will not 

 be wasted if a thorough knowledge of these idealised forms enables 

 us to understand the workings of many mechanisms that resemble 

 them only as approximations. 



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