MATHEMATICAL PHYSICS. 97 
be a general agreement that it means the reference of the 
unknown to the known; and if this be admitted, a little 
consideration will show that, if we look deep enough, science 
explaims nothing. In fact, as a rule, a scientific “ explana- 
tion” inverts the process of tracing’ the unknown to the 
known, and prefers to describe the known in terms of the 
unknown. This might be illustrated from every branch of 
science. I shall not speak of the “ affinities’’ by means of 
which chemists are wont to “explain” the reactions they 
meet with; everyone must have recognised that such terms 
are but the expression of our ignorance. I shall confine my- 
self strictly to physics, of which I am less ignorant, and 
which is generally held to be the most highly developed of 
the sciences. 
In statics we are accustomed to “explain’’ the various 
mechanical powers—the inclined plane, lever, pulleys, &c¢.— 
by reference to Newton’s parallelogram law, which is cer- 
tainly not less obscure than the thing “explained.” In 
dynamics we “‘explain’’ everything by Galileo’s or Newton’s 
laws of motion, or by the conservation of energy, or by the 
law of gravity. It is surely absurd to speak of “ explain- 
ing”’ so well-known a phenomenon as the fall of an apple 
by the statement that “every particle in the universe at- 
tracts every other particle with a force that varies directly 
as the product of the masses of the particles and inversely as 
the square of the distance between them.” Look again at 
some of the older “ explanations”’ that have now gone out 
of fashion, and are now-a-days sometimes laughed at by 
those who should know better. Take, for example, Stevinus’ 
stacement of the principle of the inclined plane. His well- 
known argument is based on what some call the “ instinctive 
knowledge”’ that an endless chain hung over an inclined 
plane will not move of itself, “ instinctive knowledge ’’ being, 
of course, an inference from experience. To-day we should 
‘explain’ this by reference to some more general “law,” 
e.g., the doctrine of energy, or the impossibility of perpetual 
motion. It seems to me that Stevinus’ statement is really 
better as an explanation. I do not deny that the modern 
method has a great advantage; but the advantage does not 
he in its superiority as an explanation. Turn from dynamics 
to sound. There we “explain” well-known phenomena by 
waves in the air, with the attendant obscurities of elasticity, 
of Boyle’s “‘law’”’ and Charles’ “law,” the adiabatic “ law,”’ 
and ail the rest. In heat, we “explain” the heating of a 
poker by the vibration of particles which no one knows by 
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