A System of Logic: Ratiocinative and Inductive, 7th Edition, Vol. I
John Stuart Mill · en
We say of a fact or statement, that it is proved, when we believe its
truth by reason of some other fact or statement from which it is said to
_follow_. Most of the propositions, whether affirmative or negative,
universal, particular, or singular, which we believe, are not believed
on their own evidence, but on the ground of something previously
assented to, from which they are said to be _inferred_. To infer a
proposition from a previous proposition or propositions; to give
credence to it, or claim credence for it, as a conclusion from something
else; is to _reason_, in the most extensive sense of the term. There is
a narrower sense, in which the name reasoning is confined to the form of
inference which is termed ratiocination, and of which the syllogism is
the general type. The reasons for not conforming to this restricted use
of the term were stated in an earlier stage of our inquiry, and
additional motives will be suggested by the considerations on which we
are now about to enter.
§ 2. In proceeding to take into consideration the cases in which
inferences can legitimately be drawn, we shall first mention some cases
in which the inference is apparent, not real; and which require notice
chiefly that they may not be confounded with cases of inference properly
so called. This occurs when the proposition ostensibly inferred from
another, appears on analysis to be merely a repetition of the same, or
part of the same, assertion, which was contained in the first. All the
cases mentioned in books of Logic as examples of æquipollency or
equivalence of propositions, are of this nature. Thus, if we were to
argue, No man is incapable of reason, for every man is rational; or, All
men are mortal, for no man is exempt from death; it would be plain that
we were not proving the proposition, but only appealing to another mode
of wording it, which may or may not be more readily comprehensible by
the hearer, or better adapted to suggest the real proof, but which
contains in itself no shadow of proof.
Another case is where, from an universal proposition, we affect to infer
another which differs from it only in being particular: as All A is B,
therefore Some A is B: No A is B, therefore Some A is not B. This, too,
is not to conclude one proposition from another, but to repeat a second
time something which had been asserted at first; with the difference,
that we do not here repeat the whole of the previous assertion, but only
an indefinite part of it.