# Is "You are an X, therefore you should know Y" a logical fallacy ?

Person A:

I wonder how to say "wheelbarrow" in French.

Person B:

C has a degree in French, so he should know.

Person C:

I have actually not paid that much attention to French, so I'm not sure.

Person B:

But you are a graduate in French, therefore you should know.

Is B's last statement a fallacy (and if so, what is it called ?)?

• The title does not match the body of the post. The example in the body describes appeal to non-authority. Its use in the last paragraph is a tactic not at all specific to this particular fallacy, and is known as poisoning the well. Oct 26, 2017 at 0:04
• Not sure that in general terms one can talk of a fallacy here. 'You are a mathematician, therefore you ought know what a logarithm is' or 'You are a plumber, therefore you ought to know why my sink is blocked' seem well-grounded 'therefores'. In these cases if S claims to be an expert, then there are certain things which S can reasonably be expected to know - 'ought to know' in one sense of the phrase. Where 'therefore' is unfairly used is in a case such as this : S is and claims to be a historian. Dec 22, 2017 at 19:40
• BTW, Fallacy is "weak logical reasoning". It's not the same thing as logical negative, i.e. false. May 22, 2018 at 12:25
• The French word for "wheelbarrow" is "brouette".
– Stef
Dec 11, 2022 at 17:08

# It is a Non Sequiteur

The statement only requires the slightest of modification to illustrate the error:

— Bob, do you know X?

— Yes I do, Alice.

— Do you know Y?

— No, I do not know Y, Alice.

— X is somewhat related to Y, Bob, and because you know X I also expected you to know Y.

— Well, Alice, I still do not know Y.

It is a straight up Non Sequiteur, because knowledge of X does not inexorably lead to knowledge of Y. Alice makes an assumption that she has no support for, at least not in terms of formal logic.

Is it reasonable for Alice to guess that Bob would know? Yes, it is reasonable. But reasonable guesses and formal logic do not always coincide, since guessing — by definition — is to make unsupported assumptions in order to reach a conclusion, which is something that clashes with formal logic.

Example: I know English fairly well, having learned and spoken it for 30+ years. But a Google search of "difficult English words" made it take less than 10 seconds before I reached "adumbrate", an English word that I did not know.

• "Is it reasonable for Alice to guess that Bob would know? Yes, it is reasonable. But reason and formal logic do not always coincide." The word "reasonable" here appears to be related to probabilities and statistics, whereas the word "reason" alone is usually not, so your jump from "reasonable" to "reason" is a bit confusing to me. "It is reasonable for Alice to guess that Bob might know Y" means: Bob's knowledge of X is evidence that Bob might have knowledge of Y. In other words: P(Bob knows Y | Bob knows X) > P(Bob knows Y). This proba inequality is weaker than a formal implication X => Y
– Stef
Dec 11, 2022 at 17:02
• So I would say "a reasonable guess does not always make a truth", or maybe "supporting evidence is not a formal proof" rather than "reason and formal logic do not always coincide".
– Stef
Dec 11, 2022 at 17:04
• @Stef Post edited. Dec 11, 2022 at 20:28

▻ FALLACY

I take this broadly as a logical mistake.

▻ NO FALLACY - REASONABLE STATEMENT

There are uses of 'You are an X, therefore you should know Y' which are perfectly reasonably. If I approached a Harvard professor of math., asked 'what is a prime number?', and they professed total ignorance, 'You are a mathematician, therefore you should know' would be completely in order in regard to so basic and elementary a matter.

▻ NO FALLACY BUT UNREASONABLE EXPECTATION

By contrast if I asked a math teacher who teaches only basic math. what Betti numbers are, this is a relatively advanced matter and it would not be reasonable to say 'You are a mathematician, therefore you should know'. The nature of Betti numbers is not something that just any mathematician can be reasonably expected to know.

▻ WHEN TO USE 'YOU ARE AN X, THEREFORE YOU SHOULD KNOW Y'

In sum 'You are an X, therefore you should know Y' is reasonable only if Y is something that falls within the standard knowledge of any and every X.

▻ THE OPEN-TEXTURE OF STANDARD KNOWLEDGE

Now the question gets interesting since it is often contestable what does fall within standard knowledge. There is often no sharp line to be drawn at which to decide where 'standard knowledge' ends - the concept is open-textured. In your linguistic example I would expect any French graduate to know the French for a 'shop' or a 'street'. But the French for 'a 'wheelbarrow' ? It's on or near the edge of everyday speech. One could spend years in France without needing to use the word (brouette, I think). As not definitely calling on standard knowledge, 'you should know' is an unreasonable expectation in the wheelbarrow case.

The description above does not seem to be about legit reasoning. What i mean by that is the speaker desires to speak down about and be negative to the person from the start. There is no formal argument present and cannot be considered a fallacy. Some people may suggest an ad hom which is an attack of the person. This is not a legit argument so we cant call a fallacy but this is rhetoric to degrade or embarass someone. Even if one is a legit expert and makes an error the backlask will come the same way. For instance "How can you be a math professor and get a muliplication problem wrong?" Rational people can make mistakes. There is no one perfect expert or non expert.

“...you are an X, therefore you should know Y”

The question is difficult because A’s inquiry (translation of wheelbarrow) is stated, but then disappears. Person B is not trying to find an answer, but rather to discredit a third person, C, by using A’s question.

I would call this statement a specialized example of an appeal to authority. Here, the intent is to first give the speaker some degree of authority based on training. Then the speaker is discredited because they are uncertain of an answer that is assumed to be within their expertise.

I didn't read B as intending to slight C. This pattern of reasoning seems legitimate when we have epistemic requirements for certain social roles — basically, people whose job is to know certain things.

Consider this exchange:
Person G: I have a cold. Will taking Vitamin C make me feel better?
Person H: J is a doctor; she should know.
Person J: I have no idea whether taking Vitamin C will help you feel better, G.
Person H: Oh. How disappointing.

J occupies a certain social role, namely, a doctor. Because J occupies this social role, is expected or socially required to know certain things, such as appropriate treatments for common illnesses. But J seems to fail to satisfy this requirement. So H's disappointment seems justified.

In the A-B-C exchange, it doesn't seem like C occupies any relevant social role, so this reasoning wouldn't apply there.

• If you are an adult, you should know how to brush your teeth.
• If you are a plumber, you should know how a toilet works.
• If you are a Professor of Mathematics, you should know High-School level algebra.

No, this is not a fallacy, nor even necessarily a false statement. The actual logical structure of the statement is merely "A implies B", which is not fallacious. As to the actual content of the statement, it is essentially an inference from a classification of a person into a type (X) to an inference about the knowledge they should have (Y). This inference might be a reasonable one, or not, but its basic structure is certainly not fallacious.

There are many statements of this form that are reasonable inferences, and are true, such as the ones I have listed above. There are other statements of this form that are not reasonable inferences, where the inferred knowledge is not something that a person of the specified type should have. As to how to draw the distinction, that is a matter of statistical inference, not philosophy per se.