An analogy points out a similarity between two things that are otherwise different. Analogies can be an effective means of arguing a point. In fact, our system of case law, which relies heavily on precedents, makes extensive use of reasoning by analogy.
A prosecutor may argue, for example, that Z is guilty because his actions resemble those of X and Y, who were judged guilty in previous rulings. In response, the defense may maintain that the actions of Z bear only a superficial resemblance to those of X and Y and that they are in fact quite different.
It is not always easy to draw the line between a reasonable and an unreasonable analogy. At times, however, an analogy is clearly off-base, in which case it is called a false analogy.
FALSE ANALOGY
If we can send a spacecraft to Pluto, we should be able to find a cure for the common cold.
How are these two ideas related? The writer has falsely assumed that because two things are alike in one respect, they must be alike in others. Exploring the outer reaches of the solar system and finding a cure for the common cold are both scientific challenges, but the problems confronting medical researchers are quite different from those solved by space scientists.