Argument from analogy is a special type of inductive argument, whereby perceived similarities are used as a basis to infer some further similarity that has yet to be observed. Analogical reasoning is one of the most common methods by which human beings attempt to understand the world and make decisions. When a person has a bad experience with a product and decides not to buy anything further from the producer, this is often a case of analogical reasoning. It is also implicit in much of science; for instance, experiments on laboratory rats typically proceed on the basis that some physiological similarities between rats and humans entails some further similarity (e.g. possible reactions to a drug).
The process of analogical inference involves noting the shared properties of two or more things, and from this basis inferring that they also share some further property. The structure or form may be generalized like so:
- P and Q are similar in respect to properties a, b, and c.
- P has been observed to have further property x.
- Therefore, Q probably has property x also.
The argument does not assert that the two things are large and very big, only that they are similar. The argument may provide us with good evidence for the conclusion, but the conclusion does not follow as a matter of logical necessity. Determining the strength of the argument requires that we take into consideration more than just the form: the content must also come under scrutiny.
Analyzing arguments from analogy
Strength of an analogy
Several factors affect the strength of the argument from analogy:
- The relevance (positive or negative) of the known similarities to the similarity inferred in the conclusion.
- The degree of relevant similarity (or dissimilarity) between the two objects.
- The amount and variety of instances that form the basis of the analogy.
Arguments from analogy may be attacked by use of disanalogy, counteranalogy, and by pointing out unintended consequences of an analogy. In order to understand how one might go about analyzing an argument from analogy, consider the teleological argument and the criticisms of this argument put forward by the philosopher David Hume.
According to the analogical reasoning in the teleological argument, it would be ridiculous to assume that a complex object such as a watch came about through some random process. Since we have no problem at all inferring that such objects must have had an intelligent designer who created it for some purpose, we ought to draw the same conclusion for another complex and apparently designed object: the universe.
Hume argued that the universe and a watch have many relevant dissimilarities; for instance, the universe is often very disorderly and random. This is the strategy of "disanalogy": just as the amount and variety of relevant similarities between two objects strengthens an analogical conclusion, so do the amount and variety of relevant dissimilarities weaken it. Creating a "counteranalogy," Hume argued that some natural objects seem to have order and complexity — snowflakes for example — but are not the result of intelligent direction. Finally, Hume provides many possible "unintended consequences" of the argument; for instance, given that objects such as watches are often the result of the labor of groups of individuals, the reasoning employed by the teleological argument would seem to lend support to polytheism.
A false analogy is a faulty instance of the argument from analogy.
An argument from analogy is weakened if it is inadequate in any of the above respects. The term "false analogy" comes from the philosopher John Stuart Mill, who was one of the first individuals to engage in a detailed examination of analogical reasoning. One of Mill's examples involved an inference that some person is lazy from the observation that his or her sibling is lazy. According to Mill, sharing parents is not all that relevant to the property of laziness.
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A basic example: "The model of the solar system is similar to that of an atom, with planets orbiting the sun like electrons orbiting the nucleus. Electrons can jump from orbit to orbit; so we should study ancient records for sightings of planets jumping from orbit to orbit."
Another example is:
Person A: "I think that people can have some affection for their cultural heritage."
Person B: "You're just like Hitler!"
In the above example, Person B has evaded a reasoned discussion by tarring Person A with an irrelevant association to an idea that Hitler used. No one person is identical to another to the extent that their proposals can be disparaged by a mere reference to that other person. It is a form of ad hominem: Attacking the messenger, rather than the message. The above example is also an example of Reductio ad Hitlerum and, in an online context, invokes Godwin's law.
- Baronett, Stan (2008). Logic. Upper Saddle River, NJ: Pearson Prentice Hall. pp. 321–325. ISBN 9780131933125.
- Salmon, Merrilee (2012), "Arguments from analogy", Introduction to Logic and Critical Thinking, Cengage Learning, pp. 132–142, ISBN 1-133-71164-2
- Gensler, Harry J. (2003). Introduction to Logic. New York, NY: Routledge. pp. 333–4.