Is reductio ad absurdum a fallacy?



If Miles told Frank: “Copying a DVD is stealing”

And Frank's response to Miles: “if copying a DVD is stealing, then, by that logic, taking a photo of someone is kidnapping”

And Miles Response is: “They are not comparable”

Is Miles right. Or is Frank's argument reasonable by making an opposing argument to contrast how absurd Miles's logic is?

Jesse Kapahua

Posted 2020-01-30T19:58:08.460

Reputation: 191

8That feels more like a "slippery slope" argument to me. – Uueerdo – 2020-01-30T20:44:39.917

6Who is right depends on one's views of intellectual property and the use of rhetorical flourishes, in any case, it is more complex than a mistake in reasoning, a.k.a. fallacy. "Stealing" and "kidnapping" are rhetorical exaggerations that are literally false, so the analogy holds at least at that level. – Conifold – 2020-01-30T21:51:57.623


Isn't that just a non sequitur? The two are not comparable and there is no logical connection between one argument (copy -> stealing) the other (picture -> kidnapping) other than the general sort of thing <action> -> <crime>. We can equally just say "eating popcorn" -> "bank robbery". It follows the same pattern but doesn't mean it's the same as the others.

– VLAZ – 2020-01-31T14:01:18.437

2In many countries people do have rights concerning having their picture taken, with exceptions defined by law. – kutschkem – 2020-01-31T14:33:59.070

1@kutschkem, do those countries define taking someone's picture as kidnapping? – CramerTV – 2020-01-31T19:09:19.183

1If something is stolen from you, you lose possession of that thing. Attempts by intellectual property holders to redefine illegal copying as theft are not logically valid. Frank is illustrating this with the conceit that photography is copying a person, so if copying an object is theft, copying a person is kidnapping. But Frank's attempt to illustrate Miles' error fails because photography only copies the appearance of a person, not the person themselves. – Medievalist – 2020-01-31T23:18:23.383

1@Medievalist I'm not sure if "only the appearence" is the main distinction between the two cases. E.g., instead of really copying a DVD (to a DVD), I might copy the files on the DVD or reencode them to a differnet audo/video format etc. – Hagen von Eitzen – 2020-02-01T22:37:02.047

Isn't this question itself an example of the fallacy of false premise? False premise: The conclusion in this conversation is an example of reductio ad absurdum. Conclusion: Since the conclusion in the example is false, reductio ad absurdum must be a fallacy. – Zano – 2020-02-02T12:59:42.227

@CramerTV - those countries define taking someone's picture w/o permission as illegal which is what copyright infringement and kidnapping are. - "Who is right depends on one's views of intellectual property" - minus one for dragging IP into this, as questions two and three. – Mazura – 2020-02-03T00:24:04.470

@CramerTV If the claim is "no harm is done by copying a DVD" then it makes no sense to counter that with an absurd example, where in reality harm is done. Taking someones picture without consent causes, or can cause, harm. – kutschkem – 2020-02-03T07:40:15.403

@Mazura, I don't think kidnapping is comparable to stealing in any way other than both are illegal. To compare taking someone's picture to depriving them of their freedom is the absurd part. – CramerTV – 2020-02-03T21:27:28.703



Reductio ad absurdum is not a fallacy. Rather, RAA is correct reasoning that exposes a fallacy. From the Logically Fallacious page for it:

[RAA is a] mode of argumentation or a form of argument in which a proposition is disproven by following its implications logically to an absurd conclusion.... The fallacy is in the argument that could be reduced to absurdity -- so in essence, reductio ad absurdum is a technique to expose the fallacy.

Mark Andrews

Posted 2020-01-30T19:58:08.460

Reputation: 4 397

7And specifically in mathematics, RAA involves establishing a contradiction. For example, the classic proof that there are infinitely many prime numbers involves showing that, if there are a finite number of primes, it's possible to construct a number that doesn't have any of them as a prime factor, and thus the complete list of prime numbers is incomplete. – Mark – 2020-01-31T08:46:46.860

7@Mark: Actually, the classical proof is direct, not by contradiction, but most people teach it and learn about it that way. Euclid showed that, in modern terms: starting from any finite set of primes, multiplying all of them and adding 1 you get a number which must have a prime factor not contained in the aforementioned set. It follows that one can construct arbitrarily long non-repeating sequences of primes, hence the set of all primes is infinite. – Vincenzo Oliva – 2020-01-31T12:54:41.873


Your example is not a valid case of Reductio ad Absurdum. It's just an example of an absurd argument.

A real example would be:

Miles: "Copying a DVD is stealing"

Frank: "Why?"

Miles: "If someone created a piece of art, they have full rights to allow or prohibit its reproduction"

Frank: "Oh, so when I take a selfie in the city, I need to obtain permission from all the architects?"

Here, Frank RAA'd the second Miles's assertion (though not the original one). This is a perfectly valid counterpoint, and Miles needs to resolve it by, e.g. pointing out the circumstances in which certain rights are granted to the public, then showing how these don't apply to DVDs.


Posted 2020-01-30T19:58:08.460

Reputation: 502


Side note: "when I take a selfie in the city, I need to obtain permission from all the architects" -> In some countries you do. Important example is the Eiffel tower at night.

– Robin – 2020-01-31T10:28:14.067

13@Robin that's the problem with Reductio ad absurdum: life gets more absurd than one's imagination. – IMil – 2020-01-31T15:23:39.920

It's not really a valid RAA of the second assertion, as a building is a piece of architectural art, not photographic art, so taking a picture of it is not making a copy of it. Unless you have a very large 3D printer. – Acccumulation – 2020-02-06T03:52:46.713

@IMil That's not a problem with RAA, it's what makes it a strong argument! – Nacht – 2020-02-07T05:31:02.100


Frank’s argument is not a reductio. It is an argument from analogy, which is not deductive reasoning and needs to be evaluated differently

(Mark’s answer adequately covers the fact that reductio is a valid form of reasoning.)


Posted 2020-01-30T19:58:08.460

Reputation: 4 599


Reductio ad adsurbum requires that there be a valid chain of reasoning that leads from the initial premise to an impossible or unacceptable conclusion.

Your example is not RAA because Frank's response does not describe an actual consequence of the Miles's statement. There may be some similarity between the two situations in Frank's mind, but Miles argues that this is tenuous.

Since the two situations are not comparable, we can't reasonably say that one leads to the other. Since the conclusion doesn't follow logically from the premise, the absurdity of the conclusion can't be used to refute the premise.


Posted 2020-01-30T19:58:08.460

Reputation: 211


tl;dr Frank's arrived at an absurd conclusion, which successfully demonstrates that one-or-more of the premises/arguments used to arrive at that conclusion is faulty. However, since Frank hasn't yet demonstrated that all of the other premises/arguments are necessarily solid, it doesn't yet follow that Miles's claim is the faulty premise. Frank's argument-as-stated is a non-sequitur.

Reductio ad absurdum attacks a system of arguments rather than a specific premise.

Reductio ad absurdum is when a system of logic is applied to arrive at an inconsistency (absurdity). Doing this demonstrates that the system of logic is inconsistent (broken).

Reductio ad absurdum can be used to attack a specific argument when it's done using only that specific argument and well-accepted premises. Then when the overall system is shown to fail, the target argument can be faulted as the other arguments are assumed to be faultless.

For example, consider the argument:

  1. If A and B, then C.

  2. If C and D, then E.

  3. E is contradicts F.

This shows that A, B, D, F, Claim 1, Claim 2, or/and Claim 3 is/are absurd. This can be taken to imply that A is absurd only if all of the others are eliminated as possible sources of error.

RE: The Frank-and-Miles scenario.

In this scenario, Frank has successfully demonstrated that whatever system of logic he used to arrive at his conclusion is broken. However, Frank has failed to demonstrate that Miles's premise is the faulty component; Frank's absurd conclusion may be due to another one of his premises or/and arguments being absurd.

So, as stated, Frank's argument is a non-sequitur, as the conclusion does not follow from the stated premises.

In common practice, there're two ways Miles might respond:

  1. Dismiss Frank's argument as a non-sequitur.
    As stated, Frank's argument is a non-sequitur. If Miles doesn't care to help develop it, Miles can just disengage.

  2. Ask for Frank to explicitly lay out his arguments.
    As stated, Frank's argument is a non-sequitur and thus invalid – but, this could be fixed if Frank lays out all of his arguments. If he does so, then the new statement of Frank's argument could be assessed.

    • If Miles can show that one of Frank's other arguments isn't solid, then Miles can demonstrate that the absurdity doesn't demonstrate the inconsistency of Miles's own argument.

    • If Frank can successfully argue his position using only arguments that Miles agrees with, then presumably Miles ought to accept that Frank has demonstrated an inconsistency in Miles's beliefs.

RE: "Sealioning".

Since reductio ad absurdum isn't an attack on a specific premise, someone making a reductio ad absurdum is putting forth an attack on the collection of all arguments that they've used in their argument, as this is strictly necessary for their conclusion that the contested premise is faulty to follow.

However, in common social situations, not everyone seems to get this. Folks might put forth a reductio ad absurdum, thinking that they're attacking only the premise that they're arguing against. Then when their other arguments are requested for critical analysis, they may feel personally attacked. This might be described as sealioning.

In such situations where an arguer isn't willing to elaborate or have their argument critically assessed, then it may be socially advisable to disengage them.

I note this here because a good reductio ad absurdum can require a lot of critical analysis to sustain, to such an extent that it's often best to just ignore such arguments.

Reductio ad absurdum is unusually non-severable.

Many logical arguments have some severability. This is, small defects don't necessarily completely invalidate an argument.

For example:

  1. 1 + 1 = 2.

  2. Humans have a left foot and a right foot.

  3. Therefore, humans have 1+1=2 feet.

  4. People walking down a sidewalk wear shoes.

  5. Therefore, people walking down a sidewalk wear 2 shoes.

Premises (2) and (4) aren't necessarily true. Still, this sort of argument is severable in that, while perhaps imperfect, it's a reasonable observation that's generally approximately true.

Now let's say that Frank and Miles are chatting. Frank lays out this argument, then appends:

  1. Look at that guy; they're not wearing any shoes!

  2. Therefore, 1+1=2 is absurd.

While we may've tacitly accepted the earlier arguments in a general context, appreciating that the conclusion in (5) is mostly correct, Frank's argument in (7) is clearly silly.

Point being that, while reductio ad absurdum is a valid mode of argument when done correctly, it's unusually non-severable. Since it's an attack on all arguments, we have to be stricter, attacking claims that, in other modes of reasoning, may've been appreciated as approximately true.


Posted 2020-01-30T19:58:08.460

Reputation: 1 788

"someone making a reductio ad absurdum is putting forth an attack on the collection of all arguments that they've used in their argument" The person making RAA is attacking their own arguments? This doesn't make any sense. – Acccumulation – 2020-02-03T00:58:41.087

@Acccumulation: RAA doesn't demonstrate that a particular premise is necessarily bad, but rather that the set of all premises/arguments/etc. is, as a collective, bad. So there's a necessary next step in which all of the other premises/arguments/etc. must be defended to establish a basis for challenging the target premise. – Nat – 2020-02-03T21:33:08.177

My point is that you're saying that the person making the RAA is attacking all of their own arguments, when presumably you mean that they are attacking all of the other person's arguments. – Acccumulation – 2020-02-03T21:38:31.460

@Acccumulation: The answer says that they're attacking "all arguments that they've used in their argument", i.e. their RAA. Were you reading it differently? – Nat – 2020-02-03T21:44:59.833

A plain reading of that sentence is that the "they" in "they are attacking" and in "they've used" refer to the same person. – Acccumulation – 2020-02-03T22:20:39.747

@Acccumulation: I agree with what you've just stated. However, I think I'm missing how that's problematic? For example, say Alice claims A, then Bob attempts RAA by arguing that A+B->C, and C is absurd; I'd say that Bob is attacking all arguments used in his own argument, i.e. {A, B, the argument that A+B yields C, and the claim that C is absurd}. Here, "all arguments that they've used in their argument" would mean "all arguments that Bob used in Bob's RAA". I'm not sure if I understand how you're reading it? – Nat – 2020-02-04T07:11:27.880


In this particular case, I don't see the comparison as valid

Let's say Alan copies Bob's DVD, then Alan takes a photo of Bob, then Alan kidnaps Bob.

In each situation, look at Alan's gain and Bob's loss and compare

Copying DVD: Bob loses nothing, Alan gains an asset (DVD) equivalent in quality to Bob's asset (DVD) (high value win for Alan, no loss to Bob) Taking a photo: Bob loses nothing, Alan gains an asset (photo) of less value than Bob (Himself) (low value win for Alan, no loss to Bob) Kidnap: Bob loses his freedom, Alan gains an asset (Bob) (high value win for Alan, high value loss to Bob)

If we equate these outcomes to scores, where 0 is unaffected, +1 is a small win, +2 is a big win, -1 is a small loss, -2 is a big loss
| BOB | ALAN | DVD | +2 | 0 | PHOTO | +1 | 0 | KIDNAP | +2 | -2 |

One could argue that a person is not of equal value to a DVD, but I equate them here because the copied DVD is of equal quality to the original, like a person who has been kidnapped, while a photo of something is not the same as having that thing.

Darren H

Posted 2020-01-30T19:58:08.460

Reputation: 101

1Why did you not include Bob stealing a DVD from Alan in your comparison? – user253751 – 2020-01-31T10:30:01.990

4flawed reasoning ... copying Bob's DVD poses a loss to the right-owner on said DVD that could have sold another copy to Alan and your table is "reversed" the values are exactly opposite of what you tell above the table – eagle275 – 2020-01-31T10:54:00.180

@user253751 because it was not included in the question. if copying a DVD is stealing, then, by that logic, taking a photo of someone is kidnapping lists 3 events for comparison – Darren H – 2020-01-31T11:03:57.797

@Darren H it lists 4: copying a DVD, stealing, taking a photo, and kidnapping. – user253751 – 2020-01-31T11:05:28.387

@eagle275 that may be the case, my comparison could have been more complete by including a third column for the rights owner, but there was no need to include that because no values in column three could alter the values in columns one and two, thus it would not affect the question of "are these equal", the answer is no, they are not equal – Darren H – 2020-01-31T11:05:40.060

3@eagle275 Copying Bob's DVD is not a loss for the rights owner. Buying a DVD is a gain for the rights owner. (If rights owners were smart, they'd say copying was legal as long as you PayPal'ed them $5 or something) – user253751 – 2020-01-31T11:06:17.187

@user253751 either way, I don't see how that would affect the conclusion – Darren H – 2020-01-31T11:06:39.420

2@DarrenH Add it to the table and you will see. Remember we are comparing two comparisons here, so four things are involved. We are not just comparing two things! – user253751 – 2020-01-31T11:07:23.097

1The rights owner of a photo might actually feel a big loss depending on what the picture shows and how it is used... – kutschkem – 2020-01-31T14:36:44.777

@kutschkem this was already covered in other comments – Darren H – 2020-01-31T15:16:57.857

When Alan takes a photo of Bob, Bob does lose something: control of his image. Quite a few cases of blackmail revolve around a photo that was taken without the consent of the persons who were photographed. Likewise, the copying of the DVD is defying the DVD's marketing companies' control over the proliferation of the content, which again is quite valuable to them. – cmaster - reinstate monica – 2020-02-01T21:51:59.650


Frank isn't engaging in RAA. They're engaging in a straw man argument. Miles simply made a claim. They didn't state what premises they have that they think support that claim. When Frank says "by that logic", there is no "that logic", so Frank is engaging in behavior that is all too common, namely simply making stuff up, and pretending that the other person said it.

Generally speaking, arguments that start off "by your logic" are not in fact arguing based on the other person's logic, they are based only simply imagining what they other person's logic might be, and then responding to that without actually trying to understand what the other person's logic really is, and don't even say what they think the other person's argument is.

If you want to make an Reductio ad Absurdum argument, you first start explicitly stating what you think the other person's logic is, and you be prepared to defend your claim that the other person is saying that. If the other person hasn't said what their logic is, then you should ask them to explain what their logic is before trying to argue against it.

If you say "A implies B", then, assuming that B is false, and the other person did in fact claim that A is an argument for C, then you have indeed presented a valid argument for why their argument for C is flawed (although you haven't presented an argument for C itself being false).


Posted 2020-01-30T19:58:08.460

Reputation: 781