MATHEMATICAL PHYSICS. 107 
doned. It may tend to clearness to illustrate this by refer- 
ence to a discarded principle of science—the horror vacua. 
As late as Galileo’s day this was the accepted “ explanation ”’ 
of many of the facts of physics. | Now, I should like to say 
that in those days it was just as good an “ explanation” as 
many of our modern laws of science. Whether we say 
that Nature abhors a vacuum, or abhors a loss of energy, we 
really explain nothing; and the horror vacui served the 
same purpose as the conservation of energy (of course, on a 
limited scale)—it co-ordinated a number of otherwise 
isolated facts. | However, it has been discarded, and rightly 
so, for it was found in Galileo’s time that the principle was 
not true. A pump was constructed with an unusually long 
pipe, and, in spite of all efforts, it would not work. Here 
was a case where Nature showed no abhorrence of a vacuum, 
and so the principle had to be abandoned. Of course, a 
similar thing might happen with some of our cherished laws; 
and possibly a few centuries hence men will laugh at our 
ideas of energy, as we have done at the horror vacui that 
gave so much satisfaction to our forefathers. 
In conclusion, it may be well to consider briefly what is 
the use of a discussion such as this. (1) In the first place, 
we cannot easily over-estimate the importance of method in 
science. It is a commonplace to dwell on the extraordinary 
advance of science within the last few centuries, compared 
with that in all the earlier ages. No one with a knowledge 
of the intellectual powers of ancient and medieval philo- 
sophers will ascribe their comparative failure to inferior 
mental capacity. © The moderns have surpassed them, not 
in mind, but in method; and hence to the moderns belongs 
the victory. With this truth before us, we can scarcely 
account it a waste of time to ponder awhile as to what the 
method of science really is. (2) Secondly, if we understand 
our method, we shall be ready to welcome hypotheses from 
every source. A harmonising principle often strikes first 
on the ear of a poet, and art grasps truth more unflinchingly 
than science. ‘‘ The rest may argue and reason—’tis we 
‘musicians know.” Philosophy, too, may help us with a 
far-reaching idea, and it is a simple matter of history that 
many important “laws” of science have been suggested by 
theological theories that scientists would not entertain. 
(3) Again, we need not be troubled about the reality of our 
hypotheses. Questions of reality are outside our province. 
Our “laws” are like the virtual images of optics; they may 
be quite unreal, but serve a very practical purpose for all 
that. Thus the trouble that some scientists have given 
