THE ART OF LOGICAL THINKING/PART 18
CHAPTER XVIII.
REASONING BY ANALOGY
What
is called Reasoning by Analogy is one of the most elementary forms of reasoning,
and the one which the majority of us most frequently employ. It is a primitive
form of hasty generalization evidencing in the natural expectation that
"things will happen as they have happened before in like
circumstances." The term as used in logic has been defined as
"Resemblance of relations; Resemblances of any kind on which an argument
falling short of induction may be founded." Brooks says: "Analogy is
that process of thought by which we infer that if two things resemble each
other in one or more particulars, they will resemble each other in some other
particular."
Jevons
states the Rule for Reasoning by Analogy, as follows: "If two
or more things resemble each other in many points, they will probably resemble
each other also in more points." Others have stated the same principle as
follows: "When one thing resembles another in known particulars, it
will resemble it also in the unknown;" and "If two things agree in
several particulars, they will also agree in other particulars."
There
is a difference between generalization by induction, and by analogy. In
inductive generalization the rule is: "What is true of the many is true of
all;" while the rule of analogy is: "things that have some things in
common have other things in common." As Jevons aptly remarks:
"Reasoning by Analogy differs only in degree from that kind of reasoning
called 'Generalization.' When many things resemble each other
in a few properties, we argue about them by Generalization. When
a few things resemble each other in many properties,
it is a case of analogy." Illustrating Analogy, we may say that if in A we
find the qualities, attributes or properties called a, b, c, d, e, f, g,
respectively, and if we find that in B the qualities, etc., called a, b, c, d, e,
respectively, are present, then we may reason by analogy that the
qualities f and g must also belong to B.
Brooks
says of this form of reasoning: "This principle is in constant application
in ordinary life and in science. A physician, in visiting a patient, says
this disease corresponds in several particulars with typhoid fever, hence it
will correspond in all particulars, and is typhoid
fever. So, when the geologist discovers a fossil animal with large, strong,
blunt claws, he infers that it procured its food by scratching or burrowing in
the earth. It was by analogy that Dr. Buckland constructed an animal from a few
fossil bones, and when subsequently the bones of the entire animal were
discovered, his construction was found to be correct." Halleck says: "In
argument or reasoning we are much aided by the habit of searching for hidden
resemblances.... The detection of such a relation cultivates thought. If we are
to succeed in argument, we must develop what some call a sixth sense of such
relations.... The study of poetry may be made very serviceable in detecting
analogies and cultivating the reasoning powers. When the poet brings clearly to
mind the change due to death, using as an illustration the caterpillar body
transformed into the butterfly spirit, moving with winged ease over flowering
meadows, he is cultivating our apprehension of relations, none the less
valuable because they are beautiful."
But
the student must be on guard against the deceptive conclusions sometimes
arising from Reasoning by Analogy. As Jevons says: "In many cases
Reasoning by Analogy is found to be a very uncertain guide. In some cases
unfortunate mistakes are made. Children are sometimes killed by gathering and
eating poisonous berries, wrongly inferring that they can be eaten, because
other berries, of a somewhat similar appearance, have been found agreeable and
harmless. Poisonous toadstools are occasionally mistaken for mushrooms,
especially by people not accustomed to gathering them. In Norway mushrooms are
seldom seen, and are not eaten; but when I once found a few there and had them
cooked at an inn, I was amused by the people of the inn, who went and collected
toadstools and wanted me to eat them also. This was clearly a case of mistaken
reasoning by analogy. Even brute animals reason in the same way in some degree.
The beaten dog fears every stick, and there are few dogs which will not run
away when you pretend to pick up a stone, even if there be no stone to
pick up." Halleck says: "Many false analogies are manufactured, and
it is excellent thought training to expose them. The majority of people think
so little that they swallow these false analogies just as newly fledged robins
swallow small stones dropped into their open mouths.... This tendency to think
as others do must be resisted somewhere along the line, or there can be no
progress." Brooks says: "The argument from Analogy is plausible, but
often deceptive. Thus to infer that since American swans are white, the
Australian swan is white, gives a false conclusion, for it is really black. So
to infer that because John Smith has a red nose and is a drunkard, then Henry
Jones who also has a red nose is also a drunkard, would be a dangerous
inference.... Conclusions of this kind drawn from analogy are frequently
fallacious."
Regarding
the Rule for Reasoning from Analogy, Jevons says: "There is no
way in which we can really assure ourselves that we are arguing safely by
analogy. The only rule that can be given is this; that the more closely two
things resemble each other, the more likely it is that they are the same in
other respects, especially in points closely connected with those observed....
In order to be clear about our conclusions, we ought in fact never to rest
satisfied with mere analogy, but ought to try to discover the general laws
governing the case. In analogy we seem to reason from one fact to another fact
without troubling ourselves either with deduction or induction. But it is only
by a kind of guess that we do so; it is not really conclusive reasoning. We
ought properly to ascertain what general laws of nature are shown to exist by
the facts observed, and then infer what will happen according to these laws....
We find that reasoning by analogy is not to be depended upon, unless we make
such an inquiry into the causes and laws of the things in question, that we
really employ inductive and deductive reasoning."
Along
the same lines, Brooks says: "The inference from analogy, like that from
induction, should be used with caution. Its conclusion must not be regarded as
certain, but merely as reaching a high degree of probability. The inference
from a part to a part, no more than from a part to the whole, is attended with
any rational necessity. To attain certainty, we must show that the principles
which lie at the root of the process are either necessary laws of thought or
necessary laws of nature; both of which are impossible. Hence analogy can
pretend to only a high degree of probability. It may even reach a large degree
of certainty, but it never reaches necessity. We must, therefore, be careful
not to accept any inference from analogy as true until it is proved to be true
by actual observation and experiment, or by such an application of induction as
to remove all reasonable doubt."
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