51 
top of the stair by one leap, but we must now descend deduc- 
tively, steadily, and methodically, trying each step, in order 
that we may establish the solidity of our footing. The 
deductive reference of any theory to every detail of the evidence 
from which it was supposed to spring cannot be too strongly 
enforced. If our law be a correct one, certain consequences 
ought to follow ; experiment or observation must search and see 
whether these consequences actually do follow ; if they do, our 
confidence is strengthened ; if not, it is in the same degree 
weakened. Newton, when meditating on the subject of 
gravity, thought it might extend as far as the moon, and at last 
guessed that she was retained in her orbit by it ; but if so, 
certain results must follow. One was that the moon must be 
deflected from the tangent every minute through a space of 
more than 15 feet; but his calculations made, so as to deter- 
mine the truth of this, gave a deflection of only 13 feet. Here 
then was discrepancy between theory and fact ; he had, pro- 
ceeding deductively, apparently proved himself wrong, by a 
small quantity indeed, but yet sufficient to induce him to give 
it up at once. But when he found he had been basing his 
calculations on a wrong magnitude of the earth, he commenced 
afresh, and now found that theory and fact agreed with remark- 
able exactness. Here then was an inference verified by evidence 
of the most satisfactory kind, and he was warranted in looking 
upon the universal prevalence of gravity as a good hypothesis. 
Because a good hypothesis is one that foretells or allows of 
deductive reasoning; that is, it must anticipate the results of 
new combinations of series of facts, prophesying the, as yet, 
unknown consequences. Another generalization was that the 
gravity of every material body is in the direct proportion of 
its mass; but if this be true, all objects, when opposing 
obstacles are removed, will fall with equal velocity. This was 
verified in the familiar experiment of the guinea and feather. 
Another important test is that there be nothing contradictory 
in the hypothesis to the known laws of nature, as ascertained 
in other departments of investigation. “ Mere difficulties of 
conception must not discredit a theory which otherwise agrees 
with facts, and we must only reject hypotheses which are incon- 
ceivable in the sense of breaking distinctly the primary laws of 
thought and nature” (Jevons). 
Then confidence in our inference is very much strengthened 
when it explains to us the meaning of evidence wholly different, 
apparently, in kind from that on which the inference is based. 
Thus the theory of the universality of gravitation, based on the 
evidence of the perturbations of the planets, was corroborated 
e 2 
