Fallacy of analogy
The fallacy of analogy is a logical fallacy that argues that because one thing resembles another in some aspects, they must resemble each other another aspect. In general, an argument by analogy takes the following form:
A is like B(they share properties X,Y, and Z). B also has property Q Therefore, A has property Q as well.
The strength of such an argument depends upon the strength of the analogy; it may actually be the case that A is not much like B at all. If A and B have more differences than shared properties, the argument is weakened. It may also be true that A is like B in many ways, but still differs from B in terms of the specific property Q. If the analogy is too weak or irrelevant to the conclusion that is drawn, then a fallacy of weak analogy has been committed. An analogy itself is never true or false; it is only weak or strong. Consider the following examples:
A tomato is like an apple. Apples grow on trees. Therefore, tomatoes grow on trees.
Here, the analogy between tomatoes and apples is weak; they may have similar size and color, but these similarities are superficial and there are many more ways in which they differ. Therefore the conclusion is not supported.
Mike and his twin Chris are the same height and weight, have similar jobs, identical political beliefs, are fans of the same sports teams, and attend the same church. Mike is male. Therefore, Chris is male also.
In this example, Mike and Chris may be virtually identical in almost all ways, and yet these similarities are not relevant to whether they are two brothers or brother and sister.
Fallacious or weak analogies are frequently used in prejudiced accusations against ethnic groups, and in polemic against religions and religious practices, and by prosecutors in courts of law to persuade judges and juries of the guilt of the accused.
A classic joke says, "If it walks like a duck, talks like a duct, and acts like a duck, it must be a duck—unless it's a politician seeking the duck vote."
