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Every syllogism has three and only three terms.
The conclusion is called the minor term; the predicate, the major term; and that with
which each is separately compared, in order to judge whether each agrees or not, the
middle term.
Every syllogism has three, and only three, propositions.
The first proposition is the major premise, in which the major term is compared with
the middle term; the second proposition is the minor premise, in which the minor is
compared, again with the middle term. And in the third proposition, the minor term is
compared with the major. An example will perhaps make the syllogistic form of
reasoning clearer:--
Light is contrary to darkness.
Sunshine is light; therefore
Sunshine is contrary to darkness.
We have chosen this particular illustration because we desire to point out one or two
possible pitfalls in argument:--
The ambiguous middle term.
Light is contrary to darkness.
Feathers are light; therefore
Feathers are contrary to darkness.
The inaccuracy arises out of the fact that the word "light" has two meanings; and
shows most forcefully the necessity for a definition of terms before an argument is begun.
The undistributed middle term.
White is a colour;
Black is a colour; therefore
Black is white.
Here the use of the word "colour" is misleading.
The illicit process of the major.
All quadrupeds are animals;
A bird is not a quadruped; therefore
It is not an animal.
This is a case of the illicit process of the major term, and the inaccuracy arises from
the employment of the whole of the term "animal" in the conclusion, when only a part of
it has been employed in the premises. "No term must be distributed in the conclusion
which was not distributed in one of the premises."