THE ART OF LOGICAL THINKING/PART 16
CHAPTER XVI.
THE SYLLOGISM
The
third and highest phase or step in reasoning—the step which follows after those
styled Conception and Judgment—is generally known by the general term "Reasoning,"
which term, however, is used to include the two precedent steps as well as the
final step itself. This step or process consists of the comparing of two
objects, persons or things, through their relation to a third object, person or
thing. As, for instance, we reason (a) that all mammals are animals; (b) that a
horse is a mammal; and (c) that, therefore, a horse is an animal.
The most fundamental principle of this step or reasoning consists in the
comparing of two objects of thought through and by means of their relation to a
third object. The natural form of expression of this process of reasoning is
called a "Syllogism."
The
process of reasoning which gives rise to the expression of the argument in the
form of a Syllogism must be understood if one wishes to form a clear
conception of the Syllogism. The process itself is very simple when plainly
stated, although the beginner is sometimes puzzled by the complicated
definitions and statements of the authorities. Let us suppose that we have
three objects, A, B and C, respectively. We wish to compare C and B, but fail
to establish a relation between them at first. We however are able to establish
a relation between A and B; and between C and A. We thus have the two
propositions (1) "A equals B; and (2) C equals A". The next step is
that of inferring that "if A equals B, and C equals A, then it must
follow, logically, that C equals B." This process is that of
indirect or mediate comparison, rather than immediate. C and B are
not compared directly or immediately, but indirectly and through the medium of
A. A is thus said to mediate between B and C.
This
process of reasoning embraces three ideas or objects of thought, in their
expression of propositions. It comprises the fundamental or elemental form of reasoning.
As Brooks says: "The simplest movement of the reasoning process is
the comparing of two objects through their relation to a third." The
result of this process is an argument expressed in what is called a Syllogism.
Whately says that: "A Syllogism is an argument expressed in strict logical
form so that its conclusiveness is manifest from the structure of the
expression alone, without any regard to the meaning of the terms." Brooks
says: "All reasoning can be and naturally is expressed in the form of the
syllogism. It applies to both inductive and deductive reasoning, and is the
form in which these processes are presented. Its importance as an instrument of
thought requires that it receive special notice."
In
order that the nature and use of the Syllogism may be clearly understood, we
can do no better than to at once present for your consideration the well-known
"Rules of the Syllogism," an understanding of which carries with it a
perfect comprehension of the Syllogism itself.
The
Rules of the Syllogism state that in order for a Syllogism to be a perfect Syllogism,
it is necessary:
I. That
there should be three, and no more than three, Propositions. These
three propositions are: (1) the Conclusion, or thing to be proved;
and (2 and 3) the Premises, or the means of proving the Conclusion, and which
are called the Major Premise and Minor Premise, respectively. We may understand
this more clearly if we will examine the following example:
Major
Premise: "Man is mortal;" (or "A is B").
Minor
Premise: "Socrates is a man;" (or "C is A").
Therefore:
Conclusion:
"Socrates is mortal" (or "C is B").
It
will be seen that the above Syllogism, whether expressed in words or symbols,
is logically valid, because the conclusion must logically follow the premises.
And, in this case, the premises being true, it must follow that the conclusion
is true. Whately says: "A Syllogism is said to be valid when the
conclusion logically follows from the premises; if the conclusion does not so
follow, the Syllogism is invalid and constitutes a Fallacy, if the error
deceives the reasoner himself; but if it is advanced with the idea of
deceiving others it constitutes a Sophism."
The
reason for Rule I is that only three propositions—a Major Premise, a Minor
Premise, and a Conclusion—are needed to form a Syllogism. If we have more
than three propositions, then we must have more than two
premises from which to draw one conclusion. The presence of more than two
premises would result in the formation of two or more Syllogisms, or else in
the failure to form a Syllogism.
II. That
there should be three and no more than three Terms. These Terms are
(1) The Predicate of the Conclusion; (2) the Subject of the Conclusion; and (3)
the Middle Term which must occur in both premises, being the connecting link in
bringing the two other Terms together in the Conclusion.
The Predicate
of the Conclusion is called the Major Term, because
it is the greatest in extension compared with its fellow terms. The Subject
of the Conclusion is called the Minor Term because it
is the smallest in extension compared with its fellow terms. The Major and
Minor Terms are called the Extremes. The Middle Term operates
between the two Extremes.
The Major
Term and the Middle Term must appear in the Major
Premise.
The Minor
Term and the Middle Term must appear in the Minor
Premise.
The Minor
Term and the Major Term must appear in the Conclusion.
Thus
we see that The Major Term must be the Predicate of the
Conclusion; the Minor Term the Subject of the Conclusion;
the Middle Term may be the Subject or Predicate of
either of the premises, but must always be found once in both
premises.
The
following example will show this arrangement more clearly:
In
the Syllogism: "Man is mortal; Socrates is a man; therefore Socrates is
mortal," we have the following arrangement: "Mortal," the Major
Term; "Socrates," the Minor Term; and "Man," the Middle
Term; as follows:
Major
Premise: "Man" (middle term) is mortal (major term).
Minor
Premise: "Socrates" (minor term) is a man (major term).
Conclusion:
"Socrates" (minor term) is mortal (major term).
The
reason for the rule that there shall be "only three" terms is
that reasoning consists in comparing two terms with each other
through the medium of a third term. There must be three
terms; if there are more than three terms, we form two
syllogisms instead of one.
III. That
one premise, at least, must be affirmative. This, because "from
two negative propositions nothing can be inferred." A negative proposition
asserts that two things differ, and if we have two propositions so asserting
difference, we can infer nothing from them. If our Syllogism stated that: (1)
"Man is not mortal;" and (2) that "Socrates
is not a man;" we could form no Conclusion, either that
Socrates was or was not mortal. There would
be no logical connection between the two premises, and therefore no Conclusion
could be deduced therefrom. Therefore, at least one premise must be
affirmative.
IV. If
one premise is negative, the conclusion must be negative. This because
"if one term agrees and another disagrees with a third term, they
must disagree with each other." Thus if our Syllogism stated that: (1)
"Man is not mortal;" and (2) that: "Socrates is
a man;" we must announce the Negative Conclusion that: (3) "Socrates
is not mortal."
V. That
the Middle Term must be distributed; (that is, taken universally) in at least
one premise. This "because, otherwise, the Major Term may be
compared with one part of the Middle Term, and the Minor Term with another part
of the latter; and there will be actually no common Middle Term, and
consequently no common ground for an inference." The violation of this
rule causes what is commonly known as "The Undistributed Middle," a
celebrated Fallacy condemned by the logicians. In the Syllogism mentioned as an
example in this chapter, the proposition "Man is mortal,"
really means "All men," that is, Man in his universal
sense. Literally the proposition is "All men are mortal," from which
it is seen that Socrates being "a man" (or some of all men)
must partake of the quality of the universal Man. If the Syllogism, instead,
read: "Some men are mortal," it would not follow that
Socrates must be mortal—he might or might not be so. Another
form of this fallacy is shown in the statement that (1) White is a color; (2)
Black is a color; hence (3) Black must be White. The two premises really mean
"White is some color; Black is some color;"
and not that either is "all colors." Another example is:
"Men are bipeds; birds are bipeds; hence, men are birds." In this
example "bipeds" is not distributed as "all bipeds"
but is simply not-distributed as "some bipeds." These
syllogisms, therefore, not being according to rule, must fail. They are not
true syllogisms, and constitute fallacies.
To
be "distributed," the Middle Term must be the Subject of a
Universal Proposition, or the Predicate of a Negative Proposition; to be "undistributed"
it must be the Subject of a Particular Proposition, or the Predicate of an
Affirmative Proposition. (See chapter on Propositions.)
VI. That
an extreme, if undistributed in a Premise, may not be distributed in the Conclusion. This
because it would be illogical and unreasonable to assert more in the
conclusion than we find in the premises. It would be most illogical to argue
that: (1) "All horses are animals; (2) no man is a horse; therefore (3) no
man is an animal." The conclusion would be invalid, because the term animal is
distributed in the conclusion, (being the predicate of a negative proposition)
while it is not distributed in the premise (being the predicate of an
affirmative proposition).
As
we have said before, any Syllogism which violates any of the above six
syllogisms is invalid and a fallacy.
There
are two additional rules which may be called derivative. Any syllogism which
violates either of these two derivative rules, also violates one or more of the
first six rules as given above in detail.
The Two
Derivative Rules of the Syllogism are as follows:
VII. That
one Premise at least must be Universal. This because "from two
particular premises no conclusion can be drawn."
VIII. That
if one premise is Particular, the Conclusion must be particular also. This
because only a universal conclusion can be drawn from two universal premises.
The
principles involved in these two Derivative Rules may be tested by stating
Syllogisms violating them. They contain the essence of the other rules, and
every syllogism which breaks them will be found to also break one or more of
the other rules given.
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