Assume there is a controversy about classification of a newly-discovered animal species. Professor A argues: if an animal has feathers, then it is a bird; this animal has feathers; therefore this animal is a bird.

Professor B has already concluded that the animal is a mammal. But instead of addressing Prof A’s argument, Prof B begins talking about the classification of fish.

Changing the subject is the simple denial that the conclusion can follow from the premises, without identifying an actual error in the reasoning. Other fallacies are clear, such as undistributed middle term or denying the antecedent. What is the fallacy here?

  • None. It could be a courteous way of indicating lack of interest or desire to avoid controversy. Even explicit denial that the conclusion follows does not require finding an actual error in the reasoning, one can have reasons to believe that the conclusion is wrong without it. For example, if someone offers a perpetuum mobile description one does not need to invest time into finding out where exactly it fails to work (although this is good sport in some cases), believing the conservation of energy law is enough. And if one wishes to avoid debates changing the subject is the right move.
    – Conifold
    Aug 22 '18 at 19:42
  • How do you get the idea that changing the subject is a statement on the original argument other than "I don't want to talk about it"? Aug 22 '18 at 22:05
  • Can a mammal be a bird too? Then it could explain the origin of these sweets. Actually, I guess the change of the subject in itself is a part of the argument, but somewhy this is never indicated in your post. If not, then how could you even conclude this behaviour to be fallacious if prof. B just does not want to discuss it?
    – rus9384
    Aug 22 '18 at 23:50
  • 1
    non sequitur
    – MmmHmm
    Aug 23 '18 at 0:45
  • @Mr.Kennedy I found a reference that supports your suggestion in my answer. (I'll delete this comment.)
    – adamaero
    Aug 23 '18 at 14:00

For any situation, generally, there can be multiple fallacies. Red herring could be a runner-up (if the scenario was altered). Ignoratio elenchi is better in this case:

The fallacy of ignoratio elenchi is committed when an argument purporting to establish a particular conclusion is instead directed to providing a different conclusion. The premises "miss the point"--the reasoning may seam plausible in itself, and yet the argument misfires ...

Introduction to Logic by Copi & Cohen, 1990. Direct link to page (may become broken). Example about tax reforms on the next page. Additionally, non sequitur is mentioned:

It may be said that every Fallacy of Relevance (except the begging of the question) is, in a sense, an ignoratio elenchi. But as we use this term, it is the fallacy in which the argument misses the point without necessarily making one of those mistakes--of false cause, or misplaced authority, or ad hominem attack, etc.--that characterize the other fallacies based on irrelevance.

The term non sequitur is also often applied...

In the future, Wikipedia may prove a quicker answer. Gold is only as good as one digging: https://en.wikipedia.org/wiki/List_of_fallacies

  • Ignoratio elenchi is so plainly the correct answer, for reasons I give in my own answer (I hadn't read yours when I wrote it), that I don't know why your answer has not been upvoted and accepted. +1 from me, anyway. GT
    – Geoffrey Thomas
    Aug 27 '18 at 9:46

Avoiding the Issue Fallacy

Molly: It is 3:00 in the morning, you are drunk, covered in lipstick, and your shirt is on backward! Would you care to explain yourself?

Rick: I was out with the guys.

Molly: And the lipstick?

Rick: You look wonderful tonight, honey!

Logically Fallacious .com

I usually call it "avoiding the topic". There are articles giving advice for how to do just that:

One way to control the conversation is to subtly change the topic when it turns to something you don’t want to talk about.


Compulsively verbal types often lacking a convincing rational argument frequently resort to this particular fallacy, avoiding the real issue.

  • It might be worth adding that this is only a fallacy if one feels that they "won" the debate by doing this. Conifold points out that there are many cases where this is politeness, rather than a fallacy.
    – Cort Ammon
    Aug 23 '18 at 1:01
  • @CortAmmon, did [s]he? As I see Conifold tells us there is no fallacy at all from the prof. B. Actually, calling everything a fallacy is weird. Even in given example there is nothing fallacious but, of course, such actions are silly as they are giving out an obvious answer.
    – rus9384
    Aug 23 '18 at 1:12

As usual Aristotle has been here before : de Sophisticis Elenchis, vi. 168a-17. The customary term is ignoratio elenchi. It's a fallacy of irrelevancy, and means literally 'ignorance of confutation'. It's a failure to address the point. You present an argument to prove X, and your opponent is guilty of ignoratio elenchi when her response is beside the point of what you are trying to prove - it proves a conclusion which is not the one needed to confute X.

'When someone responds to an argument by changing the subject, what fallacy are they using?'

I've given a formal response. Let's try an example :

A and B are discussing which is the quickest route to Zoootsville.

A : If we drive to Wampum, then head for Smithville, take a short cut through Hoburg, we should be at Zootsville by 19.00 hrs. That's three hours. I've checked the other routes and they all take four hours or more. So the Wampum/ Smithville/ Hoburg route is the quickest route.

B : But Uncle Alf, who wants to come along, hates Smithville. It has unhappy memories for him. We mustn't upset Uncle Alf.

I'm sure we all extend our sympathies to Uncle Alf but B has not confuted A's argument that the Wampum/ Smithville/ Hoburg route is the quickest route. B has not addressed the point A was seeking to establish but introduced a separate consideration entirely, quite irrelevant to A's argument and its conclusion.

  • I agree with your answer. I was also thinking this is tricky because the other person may be trying to avoid a disagreement in the first place & a change of the subject may get the original speaker to forget what he was talking about. I could also see this as a hint to “simply stop talking about this and move on to something else” as in a disagreement at a work place & a person of higher rank steps in to end the disagreement. As an authority in a work place will not let the two people go back & forth for that long. The practical idea is to “not argue back” simply redirect and move on.
    – Logikal
    Sep 3 '18 at 17:38
  • @Logikal. You've pinpointed a problem with examples. Once you scrutinise them you realise that they are always missing, or are capable of supplementation by, details (facts and counterfactuals) which make the initial analysis problematic. You've certainly scrutinised this example more deeply and fully than I did. Best - Geoffrey
    – Geoffrey Thomas
    Sep 4 '18 at 11:21

Let's break your example down to see what's going on. It's not even clear that Professor B commits any fallacy whatever as he does not state an argument (even a very bad one). Only arguments can be fallacious so the basic question should be just what the argument we are evaluating is.

1) Professor A presents a valid and sound deductive argument in the form of a material conditional."If X has feathers, then X is a bird." (P--->Q). Iff X does have feathers, then the argument amounts to a modus ponens. There is no fallacy but a simple well defined argument form used in basic Propositional Logic. The argument is valid.

2) Professor B is altogether different in your description. He a) believes --"has concluded"-- that the newly discovered animal is a mammal, b) he offers NO argument whatever, but rather c) talks about the classification of fish. This is all we're told.

At first it looks like we've got a fallacy of irrelevance here, one or another variety of a non-sequitur (red herring and ignoratio elenchi were both mentioned). But the simple fact is Professor B never makes any argument attempting to support his belief that P. We're told only that he has "already concluded" that P, and that he "talks about classification of fish." We're not given an argument, however bad, to connect the classification of fish and the conclusion that the new species is mammalian. Thus this is not even a fallacy.

Still, it is worth thinking about just what is going on here because this sort of thing happens in so-called 'arguments' in everyday life and on TV all the time. Sometimes it is purposeful distraction, as when a politician doesn't want to answer a question at a press conference and thus states something which does not even address the topic irrelevantly, but rather replaces the going topic of the questioner with a separate and new topic. Ex. Q."How many soldiers have died in the war this year?" A. "Our soldiers are extremely brave, and we honor each and every one of them. None of their heroic sacrifices are in vain, and we're going to win this war, just you watch! Next question please." Of course no answer was given, so we can't say the answer is irrelevant or poorly argued. It's simple avoidance or else mental incompetence.

Diverting topics in this way is better seen as a rhetorical device than a flaw in reasoning (unless it is done accidentally by someone with cognitive impairment). Whenever politicians are in trouble, they try to "change the narrative" or manipulate the topics that make it onto the front pages of the news. They are practiced in doing so without appearing too conspicuous. It seems that many citizens and voters do not always notice this rhetorical strategy, but it is well worth spotting whenever it occurs. Currently it is more common in public discourse than are any rational tropes!

One last point on the issue of fallacies is that there is no existing theory or even agreed-upon definition of fallacies. So even if there had been a faulty argument made in your example, classifying it with precision and to every logician's satisfaction might well be problematic. See https://plato.stanford.edu/entries/fallacies/#CurIssFalThe for discussion of that topic.

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