Reasoning by Analogy

What is reasoning by Analogy? DDT has been shown to cause cancer in rats. Therefore, there is a good chance DDT will cause cancer in humans. Rats are like humans. So if rats get cancer from DDT, so will humans. That’s arguing by analogy.

What is reasoning by Analogy? Reasoning by analogy starts with a comparison. But not every comparison is an argument. A comparison becomes reasoning by analogy when it is part of an argument: On one side of the comparison we draw a conclusion, so on the other side we conclude the same.

What is reasoning by Analogy? “My love is like a red, red rose” is a comparison but it is not an argument “Blaming soldiers for war is like blaming firemen for fires.” Analysis: This is a comparison. But it’s meant as an argument. We don’t blame firemen for fires. Firemen and fires are like soldiers and wars. Therefore, we should not blame soldiers for war.

What is reasoning by Analogy? Comparison between firemen and soldier: Firemen are involved in dangerous work. Soldiers are involved in dangerous work. The job of a fireman is to end a fire. The job of a soldier is to end a war. Firemen don’t start fires. Soldiers don’t start wars.

General Principle You shouldn’t blame someone for helping to end a disaster that could harm others, if he didn’t start the disaster.

Is the argument good? The first two premises are clearly true and so is the third. But is the job of soldiers to end war? And do soldiers really not start wars? “Without firemen there would still be fires. Without soldiers there wouldn’t be any wars.”

Is the argument good? The argument has a dubious premise. Analysis: We did not prove that soldiers should be blamed for wars. As always, when you show an argument is bad you haven’t proved the conclusion false.

Judging Analogies Magic Johnson was allowed to play in the NBA and he was HIV-positive. So people who are HIV-positive should be allowed to remain in the military.

Judging Analogies Analysis: What has NBA got to do with the military? We can state the similarities and differences but none of these matter unless we hit on the basis of the argument The general principles are very important.

Judging Analogies Premises: The only reason for eliminating someone who is HIV-positive from a job is the risk of contracting AIDS for others who work with that person. Magic Johnson was allowed to play basketball when he was HIV- positive. So in basketball the risk of contracting AIDS from a fellow worker is considered insignificant. Basketball players have as much chance of physical contact and contracting AIDS from one another as soldiers do (except in war) Therefore, the risk of contracting AIDS from a fellow soldier should be considered insignificant. Therefore, people with AIDS should be allowed to remain in the military.

Analogies in the Law Legal reasoning The basic pattern of legal reasoning is reasoning by example. It is reasoning from case to case. It is a three-step process described by the doctrine of precedent in which a proposition descriptive of the first case is made into a rule of law and then applied to the next situation. The steps are these: similarity is seen between cases; next the rule of law inherent in the first case is announced; then the rule is made applicable to the second case.

Summary Analogies: Analogies are usually incomplete arguments. Often they are best treated as motive for finding a general principle to govern our actions or beliefs by surveying similarities and differences between two cases. When a general principle is made explicit, an analogy can be a powerful form of argument. When no general principle is made explicit, an analogy can be a good place to begin a discussion.