Argument from analogy

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An argument from analogy uses an analogy (comparing objects or ideas with similar characteristics) to support an argument.

A false analogy, weak analogy, or appeal to analogy is a logical fallacy that makes a conclusion that is not supported by the analogy due, for various reasons.[1] Sometimes these differences are outright ignored by the person presenting the fallacy; other times, they may not be aware of the differences or that they apply. The fallacy occurs, and is common, because analogies are just that, analogies, and their parallels are always limited; the differences between things can often overpower their similarities. One thing people sometimes do for fun is extend a useful analogy or metaphor to the point of absurdity.

Analogies and metaphors can be very useful to explain things to people and often play an important part in learning. However, because of the prevalence of false analogies they're much less useful in making arguments.


[edit] Form

P1: A and B both have properties W, X, and Y.
P2: A has property Z.
C1: B probably has property Z.

For example:

P1: A and B are both apples, grown from the same tree, at the same time, in the same place.
P2: A is red.
C1: B is probably red.

While it's possible that B is not red, it's very likely, given the circumstances. The entirety of science (which has effectively predicted many, many events based on such analogies) is based on such arguments, especially uniformitarianism. As such, such an argument may be convincing.

It should be noted that a slightly different argument is flawed:

P1: A and B are both apples, grown from the same tree, at the same time, in the same place.
P2: A is red.
C1: B is red.

At most, this argument can prove probable similar properties, not certain properties.

[edit] Examples of flawed analogies

[edit] The "watchmaker" analogy

See the main article on this topic: Watchmaker analogy

The "watchmaker" analogy, originally formed by William Paley for the existence of God (the argument from design) and since reused as an argument for intelligent design, is cited as an example of a false analogy. In it, Paley suggested that an analogy could be made between the complexity of a watch and the complexity of the universe.[2]

The analogy is as follows:

  • The universe is like a watch.
  • A watch must have a watchmaker.
  • The universe, being like a watch, must have a designer.

The false analogy can be shown by a reduction to the absurd, highlighting the many differences between the universe and a watch. Similar absurdities can be built from almost every other characteristic of watches:

  • The universe is like a watch.
  • A watch can be used to cover a tattoo on one's wrist.
  • The universe, being like a watch, can be used to cover a tattoo on one's wrist.

Also, there are problems "disguised" in the assumptions of the analogy. A watch bears little resemblance to the universe, therefore the entire analogy is invalidated in step 1. Paley's original argument also focused on the hypothetical scenario of coming across a watch in heath, and its apparent complexity and appearance of design can be compared to stones and grass around it. Because of this, the analogy fails at a more subtle level; we have seen watches designed and made by intelligent entities and we have seen rocks and grass made by non-intelligent, natural mechanisms. The universe, however, is all we have seen—there is nothing on a similar scale and scope to the universe that implies it is non-natural.

[edit] Information argument

A similar false analogy is seen in the various forms of the information argument, such as Complex Specified Information. Generally, it goes like this:

  • DNA is a code.
  • A code requires an intelligence.
  • Therefore, DNA comes from an intelligence.

The problem with this is that the idea of DNA "encoding" the information is purely an analogy, since the DNA precedes the information rather than vice versa—rather, it could be said we lossily "encode" the information from DNA into our phenotypes, then again into our human perceptions of the same.

[edit] See also

[edit] External links

[edit] Footnotes

  1. Glenn, Cheryl; Loretta Gray. Hodges' Harbrace Handbook. 16th ed. Thomson: Boston, 2007: p. 480.

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