ry of Gravitation and the Undulatory Theory of Light as
the most conspicuous examples of such ever-victorious hypotheses. Thus,
gravitation explains the fall of bodies on the Earth, and the orbits of
the planets and their satellites; it applies to the tides, the comets,
the double stars, and gives consistency to the Nebular Hypothesis,
whence flow important geological inferences; and all this without any
need of amendment. Nevertheless, Mill, with his rigorous sense of duty,
points out, that an induction is merely a proposition concerning many
facts, and that a consilience of inductions is merely a multiplication
of the facts explained; and that, therefore, if the proof is merely
Agreement in each case, there can be no more in the totality; the
possibility of vicarious causes is not precluded; and the hypothesis
may, after all, describe an accidental circumstance.

Whewell also laid great stress upon prediction as a mark of a true
hypothesis. Thus, Astronomers predict eclipses, occultations, transits,
long beforehand with the greatest precision; and the prediction of the
place of Neptune by sheer force of deduction is one of the most
astonishing things in the history of science. Yet Mill persisted in
showing that a predicted fact is only another fact, and that it is
really not very extraordinary that an hypothesis, that happens to agree
with many known facts, should also agree with some still undiscovered.
Certainly, there seems to be some illusion in the common belief in the
probative force of prediction. Prediction surprises us, puts us off our
guard, and renders persuasion easy; in this it resembles the force of an
epigram in rhetoric. But cases can be produced in which erroneous
hypotheses have led to prediction; and Whewell himself produces them.
Thus, he says that the Ptolemaic theory was confirmed by its predicting
eclipses and other celestial phenomena, and by leading to the
construction of Tables in which the places of the heavenly bodies were
given at every moment of time. Similarly, both Newton's theory of light
and the chemical doctrine of phlogiston led to predictions which came
true.

What sound method demands in the proof of an hypothesis, then, is _not
merely that it be shown to agree with the facts, but that every other
hypothesis be excluded._ This, to be sure, may be beyond our power;
there may in some cases be no such negative proof except the exhaustion
of human ingenuity in the course of time. The