iSyy.) 
On Scientific Method. 
491 
Again, the hypothesis of Gravitation forces us to believe 
that a particle of matter here, on this earth, is at this 
moment acting upon each other particle of matter in the 
universe, and that apparently with an adtion to which time 
counts as nothing, and the mass of all the planets as a thin 
screen offering really no opposition. 
When we come to examine the hypotheses of Science, we 
find that they have been developed to very varying degrees 
of perfedtness. “ Where, as in the case of the planetary 
motions and disturbances, the forces concerned are tho- 
roughly known, the mathematical theory is absolutely true, 
and requires only analysis to work out its remotest details. 
It is thus in general far ahead of observation, and is com- 
petent to predict effects not yet even observed, as, for in- 
stance, lunar inequalities due to the adtion of Venus upon 
the Earth, &c., to which no amount of observation, unaided 
by theory, would ever have enabled us to assign the true 
cause. . . . Another class of mathematical theories, based 
to a certain extent upon experiment, is at present useful, 
and has even in certain cases pointed to new and important 
results which experiment has subsequently verified. Such 
are the dynamical theory of heat, the undulatory theory of 
light, &c A third class is well represented by the 
mathematical theories of Heat (condudtion), Bledtricity 
(statical), and Magnetism (permanent). Although we do 
not know how heat is propagated in bodies, nor what statical 
eledtricity or permanent magnetism are, the laws of their 
forces are as certainly known as that of gravitation, and can 
therefore, like it, be developed to their consequences, by the 
application of mathematical analysis.”* 
If it be impossible to group together the fadbs of Nature 
in every possible combination, and then to infer general 
laws ; if it be necessary to make use of hypotheses, it may 
be asked — Is there no method applicable for forming these 
hypotheses ? nothing to guide us in our guesses at Nature’s 
laws ? Of course it would be impossible to lay down mles 
for making hypotheses, just as it would be absurd to teach a 
man to be a genius ; nevertheless, if we study the trains of 
thought by which the most eminent naturalists have been 
led to their great discoveries, we can arrive at some general 
idea of the methods which they have followed. These dis- 
coveries have evidently been guided by analogy. From one 
similarity, or from a few similarities noticed between different 
substances or between different sets of fadts, they have 
* Thomson and Tait, The Oxford Pamphlet, p. no. 
