ters so distinctive that only one combination of causes is
possible.

The effort of science is to become less and less abstract in this
sense, by observing agencies or combinations of agencies apart and
studying the special characters of their effects. That knowledge
is then applied, on the assumption that where those characters are
present, the agent or combination of agencies has been at work. Given
an effect to be explained, it is brought home to one out of several
possible alternatives by _circumstantial evidence_.

Bacon's phrase, _Instantia Crucis_,[2] or Finger-post Instance, might
be conveniently appropriated as a technical name for a circumstance
that is decisive between rival hypotheses. This was, in effect,
proposed by Sir John Herschel,[3] who drew attention to the importance
of these crucial instances, and gave the following example: "A curious
example is given by M. Fresnel, as decisive, in his mind, of the
question between the two great opinions on the nature of light, which,
since the time of Newton and Huyghens, have divided philosophers.
When two very clean glasses are laid one on the other, if they be
not perfectly flat, but one or both in an almost imperceptible degree
convex or prominent, beautiful and vivid colours will be seen between
them; and if these be viewed through a red glass, their appearance
will be that of alternate dark and bright stripes.... Now, the
coloured stripes thus produced are explicable on both theories, and
are appealed to by both as strong confirmatory facts; but there is a
difference in one circumstance according as one or the other theory is
employed to explain them. In the case of the Huyghenian doctrine,
the intervals between the bright stripes ought to appear _absolutely
black_; in the other, _half bright_, when viewed [in a particular
manner] through a prism. This curious case of difference was tried as
soon as the opposing consequences of the two theories were noted by M.
Fresnel, and the result is stated by him to be decisive in favour
of that theory which makes light to consist in the vibrations of an
elastic medium."

III.--THE PROOF OF A HYPOTHESIS.

The completest proof of a hypothesis is when that which has been
hypothetically assumed to exist as a means of accounting for certain
phenomena is afterwards actually observed to exist or is proved by
descriptive testimony to have existed. Our argument, for example, from
internal evidence that Mill in writing h