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."