THE EXPLANATION OF LAWS 95 



foundation. Indeed, the investigator often feels that in 

 finding an explanation for the laws that he has discovered 

 he has little more latitude than in discovering those 

 laws ; it is perfectly clear from the outside what kind of 

 theory he must seek, just as it is clear what kind of law 

 he must seek. 



Thus, it may be stated broadly 1 that from 1700 to 1870 

 all physical theories were of a single type of which the 

 dynamical theory of gases which we have used as an 

 example provides an excellent instance. They were 

 all " mechanical ._" theories. In our example, the 

 behaviour of gases is explained by an analogy with a 

 piece of mechanism, a set of moving parts reacting 

 on each other with forces which determine and are 

 determined by the motion. That feature is common to 

 all the mechanical theories which played so great a 

 part in the older physics and are still prominent in the 

 newer ; ^tSey explain laws by tracing an analogy between 

 the system of which the laws are to be explained and 

 some piece of mechanisnV] Once it was realized that 

 such theories were likely to turn out to be true, the 

 task of inventing theories was greatly simplified ; it 

 often became simply that of devising a piece of mechanism 

 which would simulate the behaviour of the system of 

 which the laws were to be explained. 



But all scientific theories are not mechanical. In 

 physics it is the admission of theories that do not fall 

 within this class which distinguishes the newer from the 

 older study. And in other branches of science (except 

 where they are obviously founded on physics) theories 

 of other kinds are the rule. For instance, the theory 

 of evolution, proposed to explain the diversity and yet 

 the resemblances of different kinds of living beings is 



1 A very important exception must be made of the purely mathe- 

 matical theories, such as those of Fourier and Ampere. Some indica- 

 tion of the nature of these theories is given in Chapter VII. 



