Analogy Essay, Research Paper


Similarity in several respects between discrete cases. A logical argument

by analogy relies upon an inductive inference from the supposition that things

are similar is certain known respects to the likelihood that they are also similar in

some further unknown respect.

Example: “Jennifer enjoys listening to the music of Beethoven,

Mahler, and Bartok. Susan and Harold also like Beethoven, Mahler, and

Bartok. Chris enjoys listening to Beethoven and Mahler. Therefore, Chris

would probably like the music of Bartok, too.”

The degree of reliability achieved by such an argument depends upon the

extent and nature of the similarities that hold between the instances in its

premises and the new case in its conclusion.

Also see DPM, Keith J. Holyoak and Paul Thagard, Allison Barnes and

Paul Thagard, and Am?lie Frost Benedikt.

Analogical Reasoning

The simplest variety of inductive reasoning is argument by analogy, which takes note

of the fact that two or more things are similar in some respects and concludes that they are

probably also similar in some further respect. Not every analogy is an argument; we

frequently use such comparisons simply to explain or illustrate what we mean. But

arguments by analogy are common, too.

Suppose, for example, that I am thinking about buying a new car. I’m very likely to

speak with other people who have recently bought new cars, noting their experiences with

various makes, models, and dealers. If I discover that three of my friends have recently

bought Geo Prizms from Burg and that all three have been delighted with their purchases,

then I will conclude by analogy that if I buy a Geo Prizm from Burg, I will be delighted, too.

Evaluating Analogies

Of course, this argument is not deductively valid; it is always possible that my new car

may turn out to be an exception. But there are several considerations that clearly matter in

determining the relative strength or weakness of my inductive inference:

1.Number of instances. If five friends instead of three report their satisfaction with

the model I intend to buy, that tends to make it even more likely that I will be satisfied,

too. In general, more instances strengthen an analogy; fewer weaken it.

2.Instance variety. If my three friends bought their Prizms from three different

dealers but were all delighted, then my conclusion is somewhat more likely to be true,

no matter where I decide to buy mine. In general, the more variety there is among the

instances, the stronger the analogical argument becomes.

3.Number of similarities. If my new purchase is not only the same make and model

from the same dealer but also has the same engine, then my conclusion is more likely

to be true. In general, the more similarities there are between the instances and my

conclusion, the better for the analogical argument.

4.Relevance. Of course, the criteria we’re considering apply only if the matters with

which they are concerned are relevant to the argument. Ordinarily, for example, we

would assume that the day of the week on which a car was purchased is irrelevant to

a buyer’s satisfaction with it. But relevance is not something about which we can be

terribly precise; it is always possible in principle to tell a story in the context of which

anything may turn out to be relevant. So we just have to use our best judgment in

deciding whether or not some respect deserves to be considered.

5.Number of dissimilarities. If my friends all bought Geos with automatic

transmissions and I plan to buy a Geo with a standard transmission, then the

conclusion that I will be delighted with my purchase is a little less likely to be true. In

general, the fewer dissimilarities between instances and conclusion, the better an

analogical argument is.

6.Modesty of conclusion. If all three of my friends were delighted with their auto

purchases but I conclude only that I will be satisfied with mine, then this relatively

modest conclusion is more likely to be true. In general, arguments by analogy are

improved when their conclusions are modest with respect to their premises.

?1997, 1998, 1999 Garth Kemerling.

Questions, comments, and suggestions may be sent to:

Inductive Inferences

When an argument claims merely that the truth of its premises make it likely or

probable that its conclusion is also true, it is said to involve an inductive inference. The

standard of correctness for inductive reasoning is much more flexible. An inductive

argument succeeds if its premises provide some legitimate evidence or support for the truth

of its conclusion. Although it is therefore reasonable to accept the truth of that conclusion, it

would not be completely inconsistent to withhold judgment or even to deny it.

Inductive arguments, then, may meet their standard to a greater or to a lesser degree,

depending upon the amount of support they supply. No inductive argument is absolutely

perfect or entirely useless, although one may be said to be relatively better or worse than

another in the sense that it recommends its conclusion with a higher or lower degree of

probability. In such cases, relevant additional information often affects the reliability of an

inductive argument by providing other evidence of the likelihood of the conclusion.

It should be possible to differentiate arguments of these two sorts with some accuracy

already. Remember that deductive arguments claim to guarantee their conclusions, while

inductive arguments merely recommend theirs. Or ask yourself whether the introduction of

any additional information, without changing or denying any of the premises, could make the

conclusion seem more or less likely; if so, the pattern of reasoning is inductive.

Truth and Validity

Since deductive reasoning requires such a strong relationship between premises and

conclusion, we will spend the majority of this survey studying various patterns of deductive

inference. It is therefore worthwhile to consider the standard of correctness for deductive

arguments in some detail.

A deductive argument is valid when the inference from premises to conclusion is


If the premises of a valid argument are true, then its conclusion must also be true.

It is impossible for the conclusion of a valid argument to be false while its premises

are true.

(Considering the premises as a set of propositions, we will say that the premises are true

only when each and every one of those propositions is true.) Any deductive argument that is

not valid is invalid: it is possible for its conclusion to be false while its premises are true, so

even if the premises are true, the conclusion may be either true or false.

Notice that the validity of the inference of a deductive argument is independent of the

truth of its premises; both conditions must be met in order to be sure of the truth of the

conclusion. Out of eight distinct possible combinations here, only one is ruled out:


















The only thing that cannot happen is for a deductive argument to have true premises and a

valid inference but a false conclusion.

Some logicians designate the combination of true premises and a valid inference as a

sound argument; it is a piece of reasoning whose conclusion must be true. The trouble with

every other case, in which either one of the premises is false or the inference is invalid or

both, is that it gets us nowhere. The conclusions of such arguments may be either true or

false, so they are entirely useless in any effort to gain new information.

?1997, 1998, 1999 Garth Kemerling.

Questions, comments, and suggestions may be sent to:

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