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I should note that I'm not a formal student of philosophy and haven't studied it in any serious depth. I just like logic, and logical fallacies. I like to spot them, and I like to debate using them, primarily doing so in the (up until now) presumed belief that they improve my arguments and that, wherever a fallacy exists, so too does an invalid argument. That essentially: bad logic = bad argument.

I recently, however, came across The Fallacy Fallacy, which threw my knowledge and presumptions of what logic is upside down. What that Wiki page seems to be saying to me is that even where a claim is argued with fallacious logic, the claim itself is not necessarily wrong. I can certainly understand the reasoning behind this, but then this makes me question what the purpose of logic even is. Why employ it, if you can't use it to definitively prove to your opponent that his argument is false?

If you point out a fallacy in your opponent's argument, and they counter with a "not necessarily" in the form of the Fallacy Fallacy, where's the usefulness in logic at all? I was always under the impression that logic, as one of Russell's a priori knowledge, is a baseline of truth from which the truthfulness of all other truths can be judged, and that as a baseline, it can always be used as a yardstick. Is this not the case? Or am I just misunderstanding what the Fallacy Fallacy is?

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  • Interestingly, this question appears to be a fallacy fallacy. Personally, I don't think that logic is the baseline of truth. Truth transcends logic. Truth is paradoxical. but that does not make logic useless.
    – nir
    Dec 4, 2016 at 4:55
  • A → B | ¬A ∴ ¬B. This is the fallacy fallacy. Indeed, it can be true that ¬B sometimes, but this does not mean it always is so if you prove that premise or derivation of conclusion is false (¬A).
    – rus9384
    Aug 6, 2018 at 7:36
  • Suppose Alice and Bob are having a disagreement. Alise claims [Claim], Bob claims [Counter-claim]. A Fallacy Fallacy is if Alice says "[Claim] is true because [Argument]" and Bob then replies "No, [Argument] is a fallacy, therefore [Counter-claim] is true". Bob has assumed that just because he struck down Alice's argument, he has also struck down her claim and proved his own claim to be true. That is the Fallacy Fallacy; because Alice could still be right in her claim, she just has not shown the claim to be right yet.
    – MichaelK
    Aug 6, 2018 at 12:52
  • 1
    A fallacy is a fallacy. A logical fallacy does not prove the falsity of its conclusion, merely that it is not proven. Even if it is logically proven the conclusion might in theory, be falsified by empiricism. It's a point made by Aristotle. Logic does it's job, but logic cannot prove that Reality obeys the rules.
    – user20253
    Apr 2, 2019 at 15:10
  • (Premise: If you owned Fort Knox, you'd be rich.) Suppose I write a program which generates every possible sentence in English. Some of these will be claims about the world. Some of them will even be right - eventually it shall say "Bill Gates is rich." We may note that it has no way of knowing that - but that doesn't make it wrong, in the sense that the claim is wrong. On the other hand, I could write a program which takes claims about the world, and returns random reasons for them. I enter "Bill Gates is rich". It returns "Bill Gates is rich because he owns Fort Knox." In isolation, this sen
    – user558317
    Jun 1, 2019 at 18:07

10 Answers 10

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Short answer: definitely no, that does not make logic useless.

When someone makes an invalid argument, they're committing some sort of a formal fallacy. That is only to say that the conclusion does not logically follow from the premises. The invalidity of an argument does not say anything about the either the truth of the conclusion or the truth of the premises. So, yes, if you show that an argument is fallacious, it does not mean that its conclusion is false.

(Silly example: All men are mortal; Socrates is mortal; therefore, Socrates is a man. The premises and the conclusion are all true, but the argument is invalid.)

Why does it matter that an argument is fallacious? Here's one simple point to consider. Suppose you believe that X is true and can provide an argument for it. You then find that the argument is fallacious. Now, if that's the only argument you can come up with, then you have no reason to believe that X is true. In other words, your belief is not justified. Saying "it's still possible that X is true" is no good -- yes, it might turn out to be true, but you have no reason to believe it.

A related informal fallacy is Argument from ignorance, which claims that something is true just because it has not been proven false.

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  • Wait, maybe this is just me being completely oblivious, but why is that particular argument not valid if all the premises are assumed to be true? Nov 23, 2021 at 2:24
  • @GoodOl'SaintNick dogs are also mortal. With only the information in the two premises, Socrates might be a dog or any other variety of mortal, and so you cannot conclude from those premises that Socrates is a man. That Socrates happens to be a man does not have anything to do with the validity of the logic.
    – fectin
    Jan 24 at 18:32
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I think you may have misunderstood the nature of the Fallacy fallacy. I hope I can help a bit with that.

You are of course allowed to point out a fallacy in your opponent's reasoning. That alone does in no way correspond to a Fallacy fallacy. By doing so, you are criticizing their argument and not the claim they made. For example, I can argue that the sun is big because most people believe it is big. You can correctly point out that this is fallacious without commiting a fallacy fallacy. The moment you commit the fallacy fallacy is when you tell people that the sun is clearly not big, because I used a fallacy to argue for it.

You say:

Why employ it, if you can't use it to definitively prove to your opponent that his argument is false?

But you can prove to your opponent using logic that his argument is false. What you can't do is to infer from their bad logic, that the thing they were arguing for is actually wrong. By taking away the argument from your opponent, you are taking away reasons to believe in their claim but you are not actually disproving their claim in any way. To do that, you'd need to make non-fallacious arguments on your own.

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What that argued with fallacious logic, the claim itself is not necessarily wrong. I can certainly understand the reasoning behind this, but then this makes me question what the purpose of logic even is. Why employ it, if you can't use it to definitively prove to your opponent that his argument is false invalid?

The answer to your question is in your post. Fallacies are errors in reasoning and you can point out that someone made an invalid argument. But claims can still be true at the end of the day (through other possibly valid arguments).

If you point out a fallacy in your opponent's argument, and they counter with a "not necessarily" in the form of the Fallacy Fallacy, where's the usefulness in logic at all?

The usefulness is that you can tell them "yes necessarily, your inferences are wrong and that the conclusions don't follow from your premises because of that fallacy" assuming there was an actual fallacy committed.

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The fallacy fallacy, which is argumentum ad logicam, is the fallacy of inferring falsity from fallacy. Falsity cannot be validly inferred from fallacy.

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    Was the last word here supposed to be "fallacy"?
    – fectin
    Jan 24 at 18:32
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In Socratic dialogue format, we see how logic can be useful even with the existence of the Fallacy Fallacy:

CARL: X is false.

BILL: X is true because most people believe in it.

CARL: That's argument ad populum, a logical fallacy. Thus X is false.

BILL: That's the fallacy fallacy: just because my reasoning is faulty, doesn't make my conclusion faulty.

CARL: So we can reverse/erase the argument to before you made the logical fallacy. We are back at my statement "X is false".

BILL: Here's a mathematical proof that X is true.

CARL: This proof looks valid. Thus, I am convinced X is true.

BILL: Yes. Just because I made a mistake in arguing doesn't make me wrong. In theory, I get an unlimited number of arguments to make, and only one of them has to be valid for X to be true. No number of invalid arguments makes X false as long as there is at least one argument that makes X true.

CARL: Even if there is no argument that makes X true, it doesn't mean X is false, because it's possible that X is undecidable: there exists no proof that X is true and no proof that X is false. In other words, neither X nor NOT X can be proved.

BILL: Correct. Therefore, even if you could debunk every proof that X is true, it would not make X false.

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  • When some one says the thinking method is poor they state the FALLACY. YOU cant counter with that doesn't make my conclusion false. A fallacy indicates the conclusion does not FOLLOW from the reasons. So the conclusion may be true accidentally. That is your reasoning had nothing to do with the truth of the conclusion & why would you bring that up anyway? No one should automatically assume a conclusion is false because the argument is invalid today. Too many people are learning the topic of reasoning different from the old days. People think mayhem is logic & dont KNOW what propositions are.
    – Logikal
    Jun 20, 2019 at 23:39
  • @Logikal I think that's what I say above.
    – user935
    Jun 22, 2019 at 0:09
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The authors of forall x: Calgary Remix have this to say about arguments that are wrong (page 8):

For any argument, there are two ways that it might go wrong:

  • One or more of the premises might be false.
  • The conclusion might not follow from the premises.

To determine whether or not the premises of an argument are true is often a very important matter. However, that is normally a task best left to experts in the field: as it might be, historians, scientists, or whomever. In our role as logicians, we are more concerned with arguments in general. So we are (usually) more concerned with the second way in which arguments can go wrong.

Logic is not "usually" about whether something is true or false. That's important, but logic focuses on the methods to go from the premises to the conclusions, not whether the premises are true or false.

Like any logical fallacy, a fallacy fallacy is a faulty method of going from the premises to the conclusion. It claims that a conclusion is false because someone made an error in the method of reaching that conclusion.

To take a similar situation, suppose someone made a spelling or grammar mistake when writing a paper. That is an error. Can one conclude that because they made that typo that what they were trying to say in their paper is false? No. One cannot. That would be a faulty way to reach such a conclusion.


References

P. D. Magnus, Tim Button with additions by J. Robert Loftis remixed and revised by Aaron Thomas-Bolduc, Richard Zach, forallx Calgary Remix: An Introduction to Formal Logic, Winter 2018. http://forallx.openlogicproject.org/

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Logic is about reliability: it is about being able to count on the conclusion's truth if the premises are true. (That's "validity.") Sound arguments are valid, but all of their premises are true, so their conclusions are true.

If an argument is invalid--if its conclusion doesn't follow from its premises--then the conclusion might be true or it might be false. But you can't count on it to be true--the argument isn't reliable. Even if a conclusion is valid, it might not be sound--at least one of its premises might be false--and then you still can't count on the conclusion to be true, even though, again, it might be true.

The fallacy fallacy is about mistaking not having sufficient reason to think that the conclusion is true with having sufficient reason to think that the conclusion is false. And that's it in a nutshell.

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When we point out a fallacy or logical error in the argument, we should NOT say "... thus your thesis is false".At the most we can say "... thus I don't see any reason to believe that your thesis is true". Otherwise we will commit Fallacy Fallacy. And it's it, we can't use our knowledge of fallacies and logic to prove that our opponent has false thesis, at the most we can prove that there s/he hasn't presented us with good reasons to believe that their thesis is true. S/he can still be right, but just suck at debating.

Fallacy Fallacy doesn't make logic useless because it's still useful to tell if there are good reasons to believe something or not.

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... even where a claim is argued with fallacious logic, the claim itself is not necessarily wrong. I can certainly understand the reasoning behind this, but then this makes me question what the purpose of logic even is. Why employ it, if you can't use it to definitively prove to your opponent that his argument is false?

But it does not follow that you cannot use logic definitively to prove the falsity of an opponent's argument. All that's entailed is that a true claim can (on occasion) be defended by fallacious logic, not that all true claims are and necessarily are defended by such logic.

Take a parallel: in mathematics a true conclusion can be defended by an invalid proof. It does not follow that all true conclusions are so defended or that a true conclusion can be derived from a valid proof.

The considerations you advance do not demonstrate the uselessness of logic but only the fallibility of finite human reasoners.

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It does not. Quite the contrary.

Truth is independent of an individual's power of rhetoric and reasoning. A person can make a terrible (fallacious) case for the claim because of his/her poor reasoning, but it could be true nonetheless.

Conversely, just because some arguments are valid in logic, does not mean they are sound.

As a side note, informal logic, which deals with fallacies of critical thinking, is not the same in kind to formal logic. Many philosophers have historically contested the use of informal logic. Informal logic is more associated with practical reasoning outside the formal setting. It collects some dirty rhetorical tricks which are unfair for the speaker, but are not necessarily formally wrong. It is:

[a] collection of normative approaches to the study of reasoning in ordinary language that remain closer to the practice of argumentation than formal logic 1.

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