Elements of the philosophy of the human mind. By Dugald Stewart. Rev. and abridged, with critical and explanatory notes, for the use of colleges and schools. By Francis Bowen ...

424 REASONING AND DEDUCTIVE EVIDENCE.
essentially different from what we have in view, in any otlhe
employment of our intellectual faculties; - not to ascertinz
truths with respect to actual existences, but to trace the logical
ficiation of consequences which follow from an assumed hypothesis. If from this hypothesis we reason with correctness, nothing, it is manifest, can be wanting to complete the evidence of
the result; as this result only asserts a necessary connection
between the supposition and the conclusion. In the other
sciences, admitting that every ambiguity of language were
removed, and that every step of our deductions were rigorously
accurate, our conclusions would still be attended with more or
less of uncertainty; being ultimately founded on principles
which may, or may not, correspond exactly with the fact.
Demonstrative reasoning might be employed in the moral
sciences. - Hence, it appears, that it might be possible, by devising a set of arbitrary definitions, to form a science which,
although conversant about moral, political, or physical ideas,
should yet be as certain as geometry. It is of no moment,
whether tihe defnitions assumed correspond with facts or not,
provided they do not express impossibilities, and be not inconsistent with each other. From these principles a series of consequences may be deduced, by the most unexceptionable reasoning; and the results obtained will be perfectly analogous to
mathematical propositions. The terms true and false cannot
be applied to them; at least, in the sense in which they are applicable to propositions relative to facts. All that can be said
is, that they are, or are not, connected with the definitions which
form the principles of the science; and, therefore, if we choose
to call our conclusions true in the one case, and false in the
other, these epithets must be understood merely to refer to their
connection with the data, and not to their correspondence with
things actually existing, or with events which we expect to be
realized in future. An example of such a science as that which
I have now been describing, occurs in what has been called by
some writers theoretical mechanics; in which, from arbitrary
hypotheses concerning physical laws, the consequences are
traced which would follow, if such was really the order of
nature.