is Logic aimed at furnishing
a method for social investigations is confirmed by a letter to Miss
Caroline Fox, in which he distinctly avowed that object.

The most striking example of this crowning verification in Science
is the discovery of the planet Neptune, in which case an agent
hypothetically assumed was actually brought under the telescope as
calculated. Examples almost equally striking have occurred in the
history of the Evolution doctrine. Hypothetical ancestors with certain
peculiarities of structure have been assumed as links between living
species, and in some cases their fossils have actually been found in
the geological register.

Such triumphs of verification are necessarily rare. For the most part
the hypothetical method is applied to cases where proof by actual
observation is impossible, such as prehistoric conditions of the earth
or of life upon the earth, or conditions in the ultimate constitution
of matter that are beyond the reach of the strongest microscope.
Indeed, some would confine the word hypothesis to cases of this kind.
This, in fact, was done by Mill: hypothesis, as he defined it, was a
conjecture not completely proved, but with a large amount of evidence
in its favour. But seeing that the procedure of investigation is the
same, namely, conjecture, calculation and comparison of facts with the
calculated results, whether the agency assumed can be brought to the
test of direct observation or not, it seems better not to restrict the
word hypothesis to incompletely proved conjectures, but to apply it
simply to a conjecture made at a certain stage in whatever way it may
afterwards be verified.

In the absence of direct verification, the proof of a hypothesis is
exclusive sufficiency to explain the circumstances. The hypothesis
must account for all the circumstances, and there must be no other way
of accounting for them. Another requirement was mentioned by Newton
in a phrase about the exact meaning of which there has been some
contention. The first of his Regulae Philosophandi laid down that the
cause assumed must be a _vera causa_. "We are not," the Rule runs, "to
admit other causes of natural things than such as both are true, and
suffice for explaining their phenomena."[4]

It has been argued that the requirement of "verity" is superfluous;
that it is really included in the requirement of sufficiency; that if
a cause is sufficient to explain the phenomena it must _ipso facto_ be
the true