Weak Analogy




Also Known As: False Analogy, Fallacious Analogical Argument


This fallacy occurs when an analogical argument’s premises do not adequately support its conclusion. This is a fallacy of criteria rather than structure because a Weak Analogy and a strong argument by analogy will have the same logical form. As such, the fallacy occurs when an analogical argument fails to meet the conditions of a strong analogical argument. Analogical arguments are inductive arguments; so even a strong one with all true premises can still have a false conclusion. A related fallacy is Perfect Analogy. In this fallacy a person refuses to accept any analogy that is not perfect.

An analogical argument is an argument in which one concludes that two things are alike in a certain respect because they are alike in other respects. An analogical argument will typically have three premises and a conclusion. The first two premises establish the analogy by showing that the things (X and Y) in question are similar in certain respects (properties P, Q, R, etc.).  The third premise establishes that X has an additional quality, Z. The conclusion asserts that Y has property or feature Z as well. Although people generally present analogical arguments in an informal manner, they have the following logical form:


Premise 1: X has properties P,Q, and R.

    Premise 2: Y has properties P,Q, and R.

Premise 3: X has property Z.

Conclusion: Y has property Z.


A more concise two premise version is also common:


Premise 1: X and Y have properties P,Q,R.

Premise 2: X has property Z.

Conclusion: Y has property Z.


X and Y are variables that stand for whatever is being compared, such as chimpanzees and humans or apples and oranges. P, Q, R, and are also variables, but they stand for properties or features that X and Y are known to possess, such as having a heart. Z is also a variable, and it stands for the property or feature that X is known to possess. The use of P, Q, and R is just for the sake of the illustration-the things being compared might have many more properties in common.

An example of a non-fallacious argument by analogy presented in strict form is as follows:


Premise 1: Rats are mammals and possess a nervous system that includes a developed brain.

Premise 2: Humans are mammals possess a nervous system that includes a developed brain.

Premise 3: When exposed to the neurotoxin being tested, 90% of the rats died.

Conclusion: If exposed to the neurotoxin, 90% of humans will die.


As noted above, False Analogy is not a structural fallacy but a fallacy of criteria. To determine if an analogical argument is strong or weak enough to be fallacious, you will need to apply the standards of assessment to the argument. There can be reasonable debate about the strength of an analogical argument, and you should not automatically assume that one you disagree with must be fallacious.

The strength of an analogical argument depends on three factors. To the degree that an analogical argument meets these standards it is a strong argument. To the degree that it does not meet them, it is weak. While these standards are objective, there is no exact line at which one can say for sure that an argument would become fallacious. Fortunately, no such exact line is needed (see the Line Drawing Fallacy under the False Dilemma). Here are the three criteria for assessing analogical arguments.

First, the more properties X and Y have in common, the stronger the argument. For example, in the example given above rats and humans have many properties in common. This standard is based on the commonsense notion that the more two things are alike in other ways, the more likely it is that they will be alike in some other way. It should be noted that even if the two things are very much alike in many respects, there is still the possibility that they are not alike regarding Z.

Second, the more relevant the shared properties are to property Z, the stronger the argument. A specific property, for example P, is relevant to property Z if the presence or absence of P affects the likelihood that Z will be present. Using the example, above, the shared properties are relevant. After all, since neurotoxins work on the nervous system, the presence of a nervous system makes it more likely that something will be killed by such agents. It should be kept in mind that it is possible for X and Y to share relevant properties while Y does not actually have property Z.

Third, it must be determined whether X and Y have relevant dissimilarities as well as similarities. The more dissimilarities and the more relevant they are, the weaker the argument. In the example above, humans and rats do have dissimilarities, but most of them are probably not particularly relevant to the effects of neurotoxins. However, it would be worth considering that the size difference might be relevant and thus a difference worth considering.

While it can be tempting to label any argument by analogy you think is weak as fallacious, this temptation should be resisted. While there is not an exact line that can be drawn, you should consider whether the argument is reasonable despite your disagreement or if it fails badly enough to warrant being considered fallacious reasoning.

As an example, the watchmaker argument from design is often presented as a Weak Analogy. Oversimplified, the reasoning is that because the world is analogous to a watch, it follows that because the watch was designed by an intelligent being, the same applies to the world. While this analogy has been ably criticized by David Hume and Charles Darwin, the debate appears to be a substantial one and not settled by asserting that the argument is a False Analogy. That said, it can also be argued that it is a False Analogy.

This fallacy can be committed in good faith by someone who believes that their analogy has meet the standards. It can also be committed in bad faith when the person using it believes that the analogy is weak but presents it as if they believe it is strong. For example, a person who agrees with vaccine choice but is anti-abortion might compare vaccine choice with abortion choice in a bad faith analogy aimed at persuading a pro-choice (abortion) person to accept the vaccine choice view. Or a pro-choice (abortion) person might use this tactic against a vaccine choice person. This tactic can be used in conjunction with False Agreement and False Allegiance.

As with any fallacy, the conclusion of a Weak Analogy could be true. The error is one of reasoning and not one of fact.


Defense: The main defense against committing or falling for this fallacy is to careful apply the three standards to the argument by analogy in question. Due care should be taken before accusing someone of committing this fallacy. While you might consider their analogy weak, saying that is fallacious implies that they have made an error of reasoning that is serious enough to be called a fallacy.

If someone is committing this fallacy in bad faith, it can be useful to determine this. While the fallacy is not committed because of the bad faith, exposing it can be useful in reducing the psychological force of the fallacy. For example, if someone who is pro-choice (abortion) makes a bad faith comparison between abortion choice and vaccine choice to convince a vaccine choice person to become pro-choice (abortion), revealing the bad faith could reduce the psychological appeal of the fallacy.


Example #1

“The flow of electricity through wires is like the flowing of water through pipes. Water flows faster downhill, so electricity does, too. This, by the way, is why electrical wires are run on poles. That way the electricity can flow quickly into your house.”


Example #2

Glenn: “Biden is going to do the same things to America that Hitler did to Germany!”

Bill: “What?”

Glenn: “Biden was democratically elected. So was Hitler. Do I need to bust out some chalk and draw it out for you?”

Bill: “Yes.”

Glenn: “I’m out of chalk.”

Bill: “Too bad.”


Example #3

Steve: “Those darn Republicans!”

Lena: “How have they hurt your liberal sensibilities this time?”

Steve: “They are saying that the health care plan is a big government takeover. They are making a big lie, just like Goebbels did. It is just like blood libel.”

Lena: “That seems to be a bit much.”

Steve: “Not at all. You know, that is how the Holocaust got started. With a big lie. The Republicans are going to cause a Holocaust because they are just like the Nazis!”

Lena: “That is quite a comparison.”

Steve: “I know!”


Example #4

Ted: “While I think ghosts are cool, I don’t believe they really exist.”

Sam: “Why not?”

Ted: “Well, I have never seen one.”

Sam: “Do you believe in atoms?”

Ted: “Yeah.”

Sam: “Well, you have never seen one of them. So you should believe in ghosts if you believe in atoms.”


Example #5

Ed: “So, you are for banning guns?”

Fiona: “Yes.”

Ed: “Even for the police?”

Fiona: “Especially for the police.”

Ed: “Why ban them?”

Fiona: “Guns make it so easy to kill. Banning guns would reduce deaths.”

Ed: “So we should also ban cars?”

Fiona: “What?”

Ed: “More people are killed by people with cars than by people with guns. So, if you think we should ban guns, then you must think we should ban cars.”


Example #5

Ed: “So, you are for banning abortion?”

Fiona: “Yes.”

Ed: “Why ban them?”

Fiona: “Abortion is killing. If we banned abortion, there would be fewer deaths.”

Ed: “So we should also ban cars, guns, war, and capital punishment?”

Fiona: “What?”

Ed: “If we banned them, there would be less death. So, if you against abortion you must be against guns, car, war, and capital punishment.”

Originally appeared on A Philosopher’s Blog Read More