False analogy

A false analogy is a logical fallacy that occurs when someone applies facts from one situation to another situation but the situations are substantially different and the same conclusions cannot logically be drawn.

Sometimes these differences are outright ignored by the person presenting the fallacy; other times, they may not be aware of the differences. The fallacy occurs, and is common, because real-world parallels are always limited; the differences between things can often overpower their similarities.

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.

Banana argument

 * Bananas and telephones are both shaped to fit our hands, so bananas must, like telephones, be designed.
 * This is fallacious because a similarity in one area doesn't imply a similarity in another. For example:
 * Bananas and telephones are both shaped to fit our hands, so telephones must, like bananas, be edible.

The "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. The analogy is refuted in Richard Dawkins' book The Blind Watchmaker.

The analogy is as follows:

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:

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 that we have seen — there is nothing on a similar scale and scope to the Universe that implies it is non-natural. Thus we see that the real problem with similar arguments is: they only work if one assumes what one is attempting to prove: in this case, that the Universe is so much like a timepiece, it necessarily must have had a creator. The claim "A is similar to B" always lacks some measure of definition outside the confines of geometry. This lack of definition describes the extent that such a claim is a weak hypothesis.

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:

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 "encode" the information from DNA into our phenotypes, then again into our human perceptions of the same.