MATHEMATICAL PHYSICS. 99 
termed scientific “ explanation ” consists simply in the refer- 
ence of the facts of experience to these general principles; as 
soon as we can do this. we are satisfied (or ought to be). 
Once the real end of science is grasped, it is easy to see 
that science advances mainly by proceeding from generalisa- 
tion to wider generalisation. That this is so, and that its 
method consists in trying hypothesis after hypothesis until 
one is reached that fits in with all the facts, might be abun 
dantly proved from history. Someone should write a history 
of science from this point of view. The work would be 
important, for the history of ideas is more important than 
that of events. It should also be interesting ; most so-called 
histories miss out the most interesting parts; they tell us 
the results, but not how they were reached. The place to 
look for these things is inthe lives of the pioneers of science ; 
and it is a fascinating study to trace the growth of a great 
idea, the gradual evolution of a law, e.g., the Copernican 
theory, Kepier’s laws, Galileo’s laws of motion, Newton’s 
law of gravitation, the doctrine of the conservation of 
energy, the principle of action, the theory of the ether. Let 
me refer briefly to a few of these. We must all have been 
awed by the complex system of cycles and epicycles invented 
in the early days of astronomy to “ explain” the motions of 
the planets. Perhaps we have even felt a secret sympathy 
for the Moorish King of Castile, who, on reading an astro- 
nomical work by one of his professors, exclaimed—‘‘ Were 
the heavens thus constituted, I could have given the Deity 
good advice had He consulted me at their creation!” Then 
came Copernicus, and changed all that. He was dissatisfied 
with the current ‘“‘ explanations,’ being convinced on theo- 
logical grounds that ideal harmony and simplicity must 
characterise the motions of the heavenly bodies.- He tried 
various hypotheses, until at length he hit upon the old sug- 
gestion of the Egyptians (never followed out by them), that 
the Sun is the centre of the planetary system. He spent 
36 years in working out this idea, before he was satisfied 
that it fitted in with all his observations. Then he pub- 
lished his famous book “‘ On the Revolution of the Celestial 
Bodies.’ In the preface to this he makes a remark that is 
very relevant to the present discussion : —“‘It is not necessary 
that scientific hypotheses be either true or probable; they 
accomplish their object if they reconcile the calculus with 
observation.” Possibly he said this from motives of pru- 
dence. Considering the state of public opinion, we realise 
that he ran the risk of a heresy hunt, the consequences of 
which were more serious then than now. He may have 
