The advantage, in judging whether any controverted inference is
legitimate, of referring to a parallel case, is universally acknowledged.
But by ascending to the general proposition, we bring under our view not
one parallel case only, but all possible parallel cases at once; all cases
to which the same set of evidentiary considerations are applicable.
When, therefore, we argue from a number of known cases to another case
supposed to be analogous, it is always possible, and generally
advantageous, to divert our argument into the circuitous channel of an
induction from those known cases to a general proposition, and a
subsequent application of that general proposition to the unknown case.
This second part of the operation, which, as before observed, is
essentially a process of interpretation, will be resolvable into a
syllogism or a series of syllogisms, the majors of which will be general
propositions embracing whole classes of cases; every one of which
propositions must be true in all its extent, if the argument is
maintainable. If, therefore, any fact fairly coming within the range of
one of these general propositions, and consequently asserted by it, is
known or suspected to be other than the proposition asserts it to be, this
mode of stating the argument causes us to know or to suspect that the
original observations, which are the real grounds of our conclusion, are
not sufficient to support it. And in proportion to the greater chance of
our detecting the inconclusiveness of our evidence, will be the increased
reliance we are entitled to place in it if no such evidence of defect
shall appear.
The value, therefore, of the syllogistic form, and of the rules for using
it correctly, does not consist in their being the form and the rules
according to which our reasonings are necessarily, or even usually, made;
but in their furnishing us with a mode in which those reasonings may
always be represented, and which is admirably calculated, if they are
inconclusive, to bring their inconclusiveness to light. An induction from
particulars to generals, followed by a syllogistic process from those
generals to other particulars, is a form in which we may always state our
reasonings if we please. It is not a form in which we _must_ reason, but
it is a form in which we _may_ reason, and into which it is indispensable
to throw our reasoning, when there is any doubt of its validity: though
when the case is familiar and little complicated, and there is no
suspicion of error, we may, and do, reason at once from the known
particular cases to unknown ones.(58)