The fallacy of questionable (or false) analogy occurs when the criteria of a valid argument from analogy are lacking. These criteria are:

• similarity of the entities or relations being deemed analogous
• relevance of that similarity to the conclusion, and
• appreciable quantity and diversity of supporting examples of the elements of the analogy

Here is an example of a cogent argument from analogy:

​"Camels were rightly called 'the ships of the desert'. Therefore, stealing an importer's caravan of camels, even when not loaded, was as serious as stealing an importer's unloaded ship -- and deserving of comparable punishment."

This is potentially valid because, firstly, there is a real similarity between camels and ships -- both were used to carry mercantile goods long distances for trade between countries; and secondly, this similarity is relevant to the conclusion -- stealing either a caravan of camels or a merchant ship would inflict high financial damage to the business of traders; and finally, there is ample quantity and diversity of examples -- there are countless documented cases of ships and camels being so used.

No analogy is perfect, so of course there are some differences. For example, only a very large pack of camels could approach the cost of a typical ship. But the analogy is strong enough to serve an argumentative purpose in certain contexts.

By contrast, here is an example of a fallacy of questionable analogy:

"Not letting people have guns to protect themselves from crimes would be like not letting people have fire extinguishers to protect themselves

from fires." ​

The main problem in this argument is that guns can be used not only to prevent crimes but also to commit crimes, whereas fire extinguishers can be used to stop fires but not to start them. Since this disanalogy is relevant to the conclusion, the proposed analogy cannot serve a valid argumentative purpose here.

Notice that the following template for an argument from analogy has nothing fallacious in it:

1. A is relevantly and significantly analogous to B.

2. A is C.

3. Therefore, B is C.

The fallacy occurs when the "relevantly and significantly" part of premise 1 is not the case, as in the example of comparing guns to fire extinguishers.

A more detailed general template of this fallacy is:

1. A:B::C:D ("A is to B as C is to D").

2. A is (or has or does) E with respect to B.

3. Therefore, C is (or has or does) E with respect to D.

The above comment can be explicated as follows:

1. Fire extinguishers are to fire prevention what guns are to crime prevention.

2. Fire extinguishers should be legal in view of their use in fire prevention.

3. Therefore, guns merit being legal in view of their usefulness for crime prevention.

The above illustrates the template of this fallacy where:

A = fire extinguishers ​B = fire prevention ​C = guns ​D = crime prevention ​E = merit of being legal

Note: Just because this particular argument defending the legality of gun ownership is fallacious does not, by itself, mean that there are no other valid arguments for gun rights. To assume otherwise would be to commit the fallacy of argument from fallacy.

Image Credits: Pistol and Fire Extinguisher by Vector Portal under CC BY 4.0