Talk:Affirming the consequent

Page contents not supported in other languages.
From Wikipedia, the free encyclopedia
WikiProject iconPhilosophy: Logic Start‑class Mid‑importance
WikiProject iconThis article is within the scope of WikiProject Philosophy, a collaborative effort to improve the coverage of content related to philosophy on Wikipedia. If you would like to support the project, please visit the project page, where you can get more details on how you can help, and where you can join the general discussion about philosophy content on Wikipedia.
StartThis article has been rated as Start-class on Wikipedia's content assessment scale.
 Mid This article has been rated as Mid-importance on the project's importance scale.
Associated task forces:
Taskforce icon


Affirming the consequent is just the opposite to modus tollens, right? how is it called?

How's what called? modus tollens? Evercat 18:38 14 Jun 2003 (UTC)

I removed: If there is fire here, then there is oxygen here. (Since oxygen is required for fire.)

It was not needed and interfered with understanding the article. Also, it itself is fallacious, as it begs the question of how the sun burns, because there is no oxygen in it. -- Corey 21:31, 6 Sep 2003 (UTC)

But the sun isn't fire. It's a nuclear reaction. :P --Tiberius47 06:43, 12 December 2006 (UTC)Reply[reply]

Backwards definition[edit]

This article seems to have the definition backwards. It currently states, "The fallacy of affirming the consequent occurs when a hypothetical proposition comprising an antecedent and a consequent asserts that the truthhood of the antecedent implies the truthhood of the consequent." But in fact, the truthhood of the antecedent does imply the truthhood of the consequent. The fallacy occurs when a proposition asserts that the truthhood of the consequent implies the truthhood of the antecedent. I'm going to make the change unless someone objects. - Walkiped 01:30, 1 Dec 2004 (UTC)

Suggest 1 possible wiki link for Affirming the consequent.[edit]

An automated Wikipedia link suggester has some possible wiki link suggestions for the Affirming_the_consequent article:

  • Can link Stephen King: ...mple. Here is an argument that is obviously incorrect: :If Stephen King wrote the bible (P), then Stephen King is a good writer (Q)...

Notes: The article text has not been changed in any way; Some of these suggestions may be wrong, some may be right.
Feedback: I like it, I hate it, Please don't link toLinkBot 11:31, 1 Dec 2004 (UTC)

Popular culture examples[edit]

I recall an episode of the simpsons that involved a bear patrol using this argument as a joke, where Lisa says that the fact that there aren't any bears nearby isn't neccessarily a result of the bear patrol, just as the absence of tigers couldn't be said to be a result of a particular rock. Can the appropriate part of the script be included as an example?--Tiberius47 06:43, 12 December 2006 (UTC)Reply[reply]

Anna syllogism[edit]

Since we tend to use the word 'someone' almost exclusively in reference to humans, perhaps we could clarify Premise #1 below as follows:
If someone one is human (P), then she one is mortal (Q).
Anna is mortal (Q). Therefore Anna is human (P). --Ryan Mahoski 16:17, 12 January 2007 (UTC)Reply[reply]

Overly wordy[edit]

This entry teems with needless jargon, when the concept is a simple formal pattern. Unless someone out there can provide a good justification for verbiage such as "bidirectionality" and such, I'm going to be bold and simplify this article. 271828182 23:25, 9 February 2007 (UTC)Reply[reply]

Go ahead mate. Trim off the fat. Dbuckner 13:11, 10 February 2007 (UTC)Reply[reply]


This article should be revised to explain the difference between formal validity (which is defined in terms of the syntactic features of the argument) and the related semantic notion of validity (which is defined in terms of the move from the premises to the conclusion being a truth preserving one, or in terms of a counterfactual like, "if the premises were true, then the conclusion would be true also).

The important difference is easy to see in arguments like this: 1) If Mark Twain wrote "Huckleberry Finn" then Mark Twain is a good author. 2) Mark Twain wrote "Huckleberry Finn." 3) So, Mark Twain is a good author.

1') If Mark Twain wrote "Huckleberry Finn" then Mark Twain is a good author. 2') Samuel Clemens wrote "Huckleberry Finn." 3') So, Mark Twain is a good author.

Since Mark Twain and Samuel Clemens are the same person, the second argument is (informally) valid. That is, if its premises are true, it's conclusion is also. However, it is not formally valid because the success of the inference depends on the semantic interpretation of "Mark Twain" being the same as that of "Samuel Clemens." The first argument on the other hand, will be truth preserving under any assignment of values to terms.

This only matters insofar as the current discussion of arguments instantiating multiple argument forms is confusing, and does not adequately explain why some instances of affirming the consequent are valid arguments (that reason being that an additional but uncontroversial premise can be added to those arguments which would then make them formally valid. For instance, in the second argument above, it would be something like "Mark Twain is Samuel Clemens" or maybe, "If Samuel Clemens wrote Huckleberry Finn then Mark Twain wrote Huckleberry Finn" (which can then be used to conclude the antecedent of (1') etc. etc.

You are right to point out that the current version of the article does not distinguish between syntactic and semantic validity. In defense of the current version of the article, however, I would say it's not necessary to delve into such fine details given the basically simple nature of the fallacy. Someone who comes to this article wondering what "affirming the consequent" means would likely be baffled to no good purpose by a distinction largely irrelevant to the informal contexts in which this fallacy almost always exists. The only reason I included any mention of the technical point at all is to avoid making a statement which is flatly false, as would result if we removed the first parenthesis from the article. Also, I'd say the existing paragraph on this technicality is less confusing than, say, the explanation you've given above (no offense intended). 271828182 16:59, 8 June 2007 (UTC)Reply[reply]
Tweaked article for better (IMHO) explanations. --Kjoonlee 04:43, 9 June 2007 (UTC)Reply[reply]

Medical diagnosis[edit]

I think that there should be a mention about the fact that modern medical diagnosis relies very heavily on analysis of symptoms.

The condition causes the symptoms, The patient has the symptoms, Therefore the patient has the condition.

Of course this is vastly generalised, and medical doctors usually consider a number of possibilities, and never act until they have examined the possibilities, but they do use this to rule out a vast majority of conditions, and often a particular list of symptom is specific to a particular condition. Perhaps the limitations of this logic should be discussed in terms of real world examples.

thanks Glooper (talk) 09:00, 11 March 2008 (UTC)Reply[reply]

That's essentially "inference to the best explanation", more commonly known as (educated) guessing. I think the link to the IBE article is sufficient. If we discussed every common situation where AffCon crops up, the article would be too noisy. Besides, one of the examples is already a case of diagnostic reasoning! 271828182 (talk) 03:19, 12 March 2008 (UTC)Reply[reply]

What the hell is AffCon ? Please would EVERYONE stop using technical jargon that totally incomprehensible to the vast majority of users ? This is an encyclopedia for ordinary people, not a specialist textbook. Thanks. Darkman101 (talk) 22:09, 14 January 2011 (UTC)Reply[reply]

We should make the real world example quotes sound more like the way people actually speak.[edit]

The article says:


Arguments of the same form can sometimes seem superficially convincing, as in the following example:

If I have the flu, then I have a sore throat. I have a sore throat. Therefore, I have the flu.


But teaching people how this argument will really come up in the real world should read like this:


Every time I have the flu, it always makes my throat sore. I have a sore throat today, so that means I must have the flu.


Not being an expert in this field, I don't want to edit the actual article. But if my suggestion doesn't violate the rules of the fallacy, then please add it to the article. —Preceding unsigned comment added by (talk) 19:01, 17 August 2008 (UTC)Reply[reply]

isn't this whole article trying to deal with 3 fallacies as one?

1.converse accident


3.division —Preceding unsigned comment added by (talk) 15:48, 2 July 2009 (UTC)Reply[reply]

concise vs. flabby prose[edit]

I don't see how having more than a couple examples makes the article better. If someone is coming to this article, they want or need an explanation of a basic concept in informal reasoning. Making it longer than necessary serves no clear purpose except to waste time. If you want a reference, try Strunk & White, The Elements of Style, or Orwell, "Politics and the English Language". I will note that User: Loodog has offered no justification for including the extra words besides saying "it's better". 271828182 (talk) 18:01, 11 September 2009 (UTC)Reply[reply]

It's clearer. The reason that p->q does not necessitate that q->p is not obvious to most people. And it perfectly sets up the later section about if and only if.--Louiedog (talk) 19:01, 11 September 2009 (UTC)Reply[reply]
It's not only less concise, it's wrong. A conditional is not necessarily a causal statement, which the current explanation implies. 271828182 (talk) 23:22, 11 September 2009 (UTC)Reply[reply]
Fixed.--Louiedog (talk) 16:00, 12 September 2009 (UTC)Reply[reply]

Added another example of the fallacy[edit]

Examples are one of the best means for the reader to understand fallacious reasoning. I added an real world example which shows that the logical fallacy of affirming the consequent is a very common one and is sometimes committed by educated people. The example I added is this:

If the mind is the activity of the brain, then any serious change in the brain (from strokes, drugs, electricity or surgery) will affect the mind.
Any serious change in the brain does affect the mind.
Therefore the mind is the activity of the brain.

This is a clear case of the fallacy of affirming the consequent, and is thus a valid as well as a realistic example.

This is the second time I add this example. The first time user Loodog removed it arguing thus: "source says (1) NOTHING about brain produces mind argument, or (2) it being a form of this fallacy. example is also not the clearest b/c brain damage affects mind is circumstantial evd".

The source at hand is this Newsweek article which nicely demonstrates the fallacy at work in the real world. It's not true that this article says nothing about the brain producing the mind; indeed its very subtitle is "The soul is the activity of the brain" and in the article "soul" and "mind" are used interchangeably. In any case, just in order to use the same language of the source, I have now substituted the clause "your brain produces your mind" with "the mind is the activity of the brain".

Loodog further claims that my example is not the clearest because the fact that brain damage affects the mind is at least circumstantial evidence. But having a sore throat is circumstantial evidence for having the flu also, but this does not imply that the second example in the article is not a clear one. The very reason why affirming the consequent is fallacious is because people fail to consider the possibility of other alternative explanations. One can find "circumstantial evidence" for about anything one wishes, but this does not make the fallacy of affirming the consequent any less fallacious. Dianelos (talk) 13:43, 11 July 2010 (UTC)Reply[reply]

Then you're using the source to argue the very opposite of what it says. Sources should cite supporting evidence, not examples of contradiction. That's original research and it would be equivalent to me citing the IPCC's statement on climate change in an article about scientific fallacy. Additionally, the fact that brain affects mind therefore brain makes mind even is affirming the consequent is original research.
Also, "people today find convincing" is has POV issues, implying that it's wrong from the get-go.
This not the best example, again, because the conclusion is generally accepted as likely enough in science to be correct. Good examples are those like Bill Gates owns Fort Knox because they are very clearly fallacious and also have conclusions that are flagrantly wrong. Also, you mention the fallacy as being relevant because it cuts off investigation into alternative explanations. In the mind-brain example you give, you've offered no alternative explanation to consider, again making it a not so illustrative example.--Louiedog (talk) 16:11, 11 July 2010 (UTC)Reply[reply]
I used the source to demonstrate what I claimed, namely that many people are convinced by the argument I presented as an example. If you read that Newsweek article you’ll see that the only argument made there was the one I was presenting, and which is a clear case of affirming the consequent. That people find this argument convincing is not my POV; the very source I quote proves this.
The fact that the conclusion of the argument is generally accepted as likely enough in science, is entirely irrelevant. Fallacious reasoning is *not* the reasoning that leads to a false conclusion. Indeed it is very well possible that the conclusion of a fallacious argument is true; this does not make the logic of the argument any less fallacious. So, to use the second example in the article, it may be true that I have the flu, but this does not help diminish that argument’s fallacious logic. A formal fallacy, such as affirming the consequent, is a function of the structure (or the syntax) of an argument, not of its semantic content. To come back to the present example, it may well be true that “the mind is the activity of the brain”. Many people, indeed many brain scientists, believe that. This does not make the argument I use as an example any less fallacious. In general, what makes an argument fallacious is that the truth of its conclusion is not established on the truth of its premises, and therefore its conclusion *may* be false even if the premises are true.
I agree that a good article should include some examples of arguments where the conclusion is fragrantly wrong. But I think it should also include some examples where the truth or falsity of the conclusion is not clear, because knowledge about logical fallacies is useful precisely in such cases. Nobody needs to know about logical fallacies for understanding that the premises do not establish the conclusion “Bill Gates owns Fort Knox”, for everybody already knows that Bill Gates does not own Fort Knox. Rather, one needs to know about the logical fallacy of affirming the consequent for realizing that the conclusion “The mind is the activity of the brain” is not established by the premises of that argument, even though many people, including scientists, feel that the premises do establish this conclusion. If one understands the fallacious nature of that argument, one will see that more evidence is needed before one is justified in believing its conclusion.
I did mention an alternative explanation, namely substance dualism, which explains the fact that a serious change in the brain affects the mind, while positing that the mind is not the activity of the brain. In philosophy there are many other alternative explanations for the same fact, for example Descartes’ evil demon hypothesis, Berkeley’s subjective idealism, or, more recently, Nick Bostrom’s computer simulation hypothesis.
In conclusion then I think that 1) the argument I gave is a clear example of committing the fallacy of affirming the consequent, and 2) it is a real world example of fallacious thinking which many people commit. Therefore I think its inclusion will raise the reader’s consciousness about how important it is to be careful and check the logic of the arguments on which one’s beliefs rest.
It's not really relevant to our discussion, but I think it is interesting to note that, contrary to the impression that many people have, it’s really not the case that we know for certain, or even have good reason to believe, that “the mind is the activity of the brain”. Sam Harris, who is trained both in philosophy and in neuroscience, writes the following in his “The End of Faith”: “The idea that brains produce consciousness is little more than an article of faith among scientists at present, and there are many reasons to believe that the methods of science will be insufficient to either prove or disprove it”. So he is not denying that most scientists believe that, but he knows that the evidence is not really there and, moreover, that such evidence may never be forthcoming. Another way to see the same is this: If we had good evidence for the claim that the mind is the activity of the brain, then somebody would have used it to falsify Nick Bostrom's computer simulation hypothesis; but nobody has, so we don't have such good evidence. (Incidentally, the preceding argument is one that *denies* the consequent, and is thus a valid syllogism: If the premises are true then it is established that the conclusion is true also.) Dianelos (talk) 00:27, 12 July 2010 (UTC)Reply[reply]
The computer simulation hypothesis, Descartes' Evil Demon, and subjective idealism are all essentially hard skepticism, which makes the example given all the more esoteric. The examples given so far are so common sense, a background in science isn't even needed. The example you give suggests than the "brain creates the mind" hypothesis could be wrong if you throw all science (or even just all perception) into dispute, which, aside from being obscure, is far from intuitive or easily understood. If the point is to include an example where the conclusion is correct in spite of the fallacy, there are far simpler examples to choose from:
1. If the grass is alive, then it will be green.
2. The grass is green; therefore, the grass is alive.
And on your aside, the computer simulation hypothesis is not falsifiable, nor is it incompatible with the brain creates mind hypothesis, so its failure to be falsified is hardly evidence against the operating tenet of such fields as neuroscience and psychiatry. You can say the MRI machine is just a contrivance of the mind that doesn't really exist, but while you're at it, you've thrown out every empirical tool that exists, leaving only human logic, which is just as susceptible to "oh nothing exists, nothing can be trusted!" as anything else. A Evil Demon is leading your consciousness to logical nonsense for his own entertainment and the very concept of existence is a complete hallucination subjectively experienced by itself.
--Louiedog (talk) 17:25, 13 July 2010 (UTC)Reply[reply]
A logically valid argument is one that guarantees that if the premises are true then the conclusion is true also. That’s all there is to formal logic. Formal logic has absolutely nothing to do with whether a hypothesis is falsifiable or not, or with the operating tenets of neuroscience and psychiatry, or with the relevance of empirical tools, or with what scientific consensus is, or with any other opinion about epistemological principles you or I or anybody else may hold. Indeed a logically valid argument does not guarantee that its conclusion is true (never mind that it will be “scientific”). Here’s an example of a logically valid argument (modus ponens) with a false conclusion :
1. If Bill Gates is rich then he owns Fort Knox.
2. Bill Gates is rich.
3. Therefore Bill Gates owns Fort Knox.
As the article in its introduction makes clear, a logically invalid argument is one where even if the premises are true the conclusion can be false. Still, the conclusion of a logically invalid argument may also be true. Here’s an example of a logical fallacy (affirming the consequent), which nevertheless leads to a true conclusion:
1. If Bill Gates is a millionaire then he is rich.
2. Bill Gates is rich.
3. Therefore Bill Gates is a millionaire.
As for what examples an encyclopedia should include, I’d say that it is not a good idea to only include simple examples, where people immediately see that there is something amiss even without knowing anything about logic. Rather, it is educational to include realistic examples where the logic is invalid even though the argument appears to be a good one. Common sense often leads people to fallacious logic, and that’s precisely why formal rules of logic are needed.
Incidentally, it is not true that the computer simulation hypothesis is compatible with the brain creates mind hypothesis, because according to the computer simulation hypothesis our brain does not even objectively exist, never mind creates our mind. But in any case, again, these issues are irrelevant. It’s not like the invalid logic of affirming the consequent becomes valid if one judges that all other alternative explanations are “hard skepticism” or “not falsifiable” or “not scientific” or whatever. One does not even have to be aware of any alternative explanations. The logical fallacy lies in the structure of the argument, not in one’s judgment of its premises. As the article makes explicitly clear in the context of the use of such reasoning in science “such reasoning is still affirming the consequent and logically invalid”.
By the way, here’s another realistic example of invalid logic that many people, including scientists, fall for:
1. If natural evolution is not guided by some intelligence then it will produce organisms with design errors (e.g. a dangerously narrow birth canal, an appendix attached to the large intestine which can easily get infected, etc). [true premise]
2. Natural evolution has produced organisms with design errors. [true premise]
3. Therefore natural evolution is not guided by some intelligence. [this conclusion does not logically follow from the premises, and therefore we can’t know whether it is true or false based on the premises.] Dianelos (talk) 22:23, 22 July 2010 (UTC)Reply[reply]
1. If Bill Gates is a millionaire then he is rich.
2. Bill Gates is rich.
3. Therefore Bill Gates is a millionaire.
Yes, this is a good example of an affirming the consequent fallacy where the conclusion is correct that doesn't require delving into philosophical erudition. I would support including it.
(1)Incidentally, it is not true that the computer simulation hypothesis is compatible with the brain creates mind hypothesis, because according to the computer simulation hypothesis our brain does not even objectively exist, never mind creates our mind.
(2) If we had good evidence for the claim that the mind is the activity of the brain, then somebody would have used it to falsify Nick Bostrom's computer simulation hypothesis; but nobody has, so we don't have such good evidence.
I took your (2) as an actual argument that the computer simulation should be taken seriously, not as an unrelated example of valid logic. I therefore argued not with the the form of the argument (which is valid), but with its soundness. If you just meant it as an unrelated piece of logic to contemplate, OF COURSE, it's fine and the overwhelming scientific consensus doesn't change that it's valid in form. If you're seriously arguing that the fact that the computer simulation hypothesis hasn't been falsified is evidence against the brain creates mind hypothesis, you're mistaken.
Incidentally, they're not incompatible in any empirical sense. Every observation that supports the brain creates mind hypothesis could also be dismissed as "that observation is just an observation inside a computer simulation." That you could conduct gravitational experiments inside The Matrix doesn't mean any of them are incompatible with being inside The Matrix.
And your design errors aside is covered in the article under the use in science section. It's logically invalid, but observations are used as evidence that the hypothesis is more likely to be correct. For this reason, no hypothesis of experimental science is ever, strictly speaking, proven; hence, Einstein's famous quote, "No amount of experimentation can ever prove me right; a single experiment can prove me wrong."--Louiedog (talk) 02:15, 23 July 2010 (UTC)Reply[reply]

Use of the fallacy in science--climate change example[edit]

The climate change example is an oversimplification of the scientific arguments underlying climate research. Even if the example as written is logically correct, it is factually false or at best misleading because it clearly implies that climate scientists simply looked at two sets of data (global temperature increases and CO2 emissions increases) and drew a definitive conclusion and closed the case. The hypothesis is that increases in carbon dioxide affect the temperature of the earth. Humans produce prodigious amounts of carbon dioxide and that there is a distinct and clear correlation between the increase in carbon dioxide emissions and the increase in global temperatures. No other viable competing cause can be found to explain the increase in global temperatures (asteroid hit, sudden, large amounts of volcanic activity). Where is all the CO2 coming from? If this is the logical fallacy of affirming the consequent, then so is placing a pot of water on the stove, turning on the burner and predicting that the consequent boiling of water affirms that the heat of the burner is the cause. If you are going to use a real-world example, it should be a sound one, and not one that promotes a political agenda (whether intended or not).

General Ludd (talk) 16:02, 4 August 2010 (UTC)Reply[reply]

Absolutely right. Global warming is not much contested in the scientific consensus, and is an awfully political issue that detracts from the content of the logical argument. Since all of science essentially relies on affirming the consequent, better to apply it to something less contested politically. I've changed the example to General Relativity and the observed Mercury Perihelion shift.--Louiedog (talk) 18:44, 4 August 2010 (UTC)Reply[reply]

Louiedog, please would you change that example, as it is totally incomprehensible to the vast majority of people, who don't have the faintest understanding of General Relativity ? I am a reasonably well educated person, and it certainly makes no sense to me. Thanks. Darkman101 (talk) 22:30, 14 January 2011 (UTC)Reply[reply]

Why not give a simple example requiring no specialist scientific knowledge. The purpose of examples is to make a point easier to understand, not to introduce additional complexities Philogo (talk) 01:43, 15 January 2011 (UTC)Reply[reply]
if you look at Scientific method#Introduction to scientific method you will see that scientific metnod is CONTRASTED with affirming the consequent. After all it would strange, to say the least, were scientific advance be based upon the application of a fallacy.

Philogo (talk) 02:44, 15 January 2011 (UTC)Reply[reply]


I do not think that the following part of the article is correct:-

If claims P and Q express the same proposition, then the argument would be trivially valid, as it would beg the question.

An argument schema is said to be valid if and only if the premises cannot be true but the conclusion false. That it to say the schema is valid if the Corresponding conditional (logic) is a logical truth. It does not make a lot of sense to say an argument scheme is valid "when" the premises have a given interpretation.

Consider the argument schema P therefore Q. This is clearly invalid becasue it can have a true premiss but a false concusion. It would be surely to misu----Philogo (talk) 14:31, 14 November 2010 (UTC)nderstand the meaning of the term "valid" to say that the schema (P therefore Q) would be trivially valid if P and Q express the same proposition. If we provide a missing premiss then we can get a valid schema, i.e. P iff Q, P Therefore Q.Reply[reply]

Similarly the schema P iff Q, If P then Q, Q Therefore P is Valid (as is the simpler P iff Q, Q Therefore P. Note that what we are doing is providing as the missing premiss the supposition P and Q express the same proposition by means of P iff Q --Philogo (talk) 18:15, 13 November 2010 (UTC)Reply[reply]

I suspect the confusion has crept in after numerous edits by disparate hands. The sentence you quote is indeed incorrect if you take "the argument" to refer to "the argument schema", but the writer probably meant an argument token. The whole article has become infested with weeds and mold and should be cleaned up and shortened. 271828182 (talk) 23:57, 13 November 2010 (UTC)Reply[reply]
I suggest the deletion of the para Tautologies and the follwoing if and only ifPhilogo (talk) 14:31, 14 November 2010 (UTC)Reply[reply]
Have now deleted offending txtPhilogo (talk) 20:02, 3 December 2010 (UTC)Reply[reply]

Use of the fallacy in science[edit]

This section is really very bad. It's an implied argument between a fallacious deduction, and probabilistic reasoning. There's a much better page on it, abductive reasoning. Maybe a watcher of the page wants to simply kill that argumentative and uncited section that sounds like a freshmen philosophy 101 argument, and just reference abductive reasoning as it's usage in science? —Preceding unsigned comment added by (talk) 21:47, 27 January 2011 (UTC) Too true: section deletedPhilogo (talk) 16:14, 28 January 2011 (UTC)Reply[reply]

I don't follow what you mean. The fallacy is used in science while the argument committing it is still given credence. I thought the section gave a nice explanation of why this is so and was necessary.--Louiedog (talk) 17:41, 28 January 2011 (UTC)Reply[reply]
The section failed to give any example of the use of the fallacy in any scientific publication, nor cite any source saying that it was any part of scientific method. Wiki's own articleScientific Method#Introduction to scientific method expressly states that the use of the fallacy would be an error, i.e. not part of scientific method. It is a common misudedrstanding of scientific method; the section gave a false account of scientific method and did nothing to elucidate the subject of the article.Philogo (talk) 23:22, 29 January 2011 (UTC)Reply[reply]
One famous example from a scientific publication: The Origin of Species, pp. 293f. (1900 edition): "It can hardly be supposed that a false theory would explain, in so satisfactory a manner as does the theory of natural selection, the several large classes of facts above specified. It has recently been objected that this is an unsafe method of arguing; but it is a method used in judging of the common events of life, and has often been used by the greatest natural philosophers. The undulatory theory of light has thus been arrived at; and the belief in the revolution of the earth on its own axis was until lately supported by hardly any direct evidence."
The text does not say or demonstrate that Affirming the consequent was or is used in scientific method. One more please read Wiki's own article at Scientific Method#Introduction to scientific method If an editor has sources to cite in support of this then they would belong in the article Scientific Methodnot in this article. Philogo (talk) 17:05, 31 January 2011 (UTC)Reply[reply]
The example from Darwin is an example of inference to the best explanation. --ExperiencedArticleFixer (talk) 11:31, 12 November 2018 (UTC)Reply[reply]

Other examples[edit]

An example I heard from a college philosophy class:

  • Caterpillars eat cabbage.
    Catherine eats cabbage.
    Therefore, Catherine is a caterpillar.

This same "logic" is regularly used to assign incorrect labels and intentions, usually moral or political, to individuals and groups:

  • The Nazis implemented strict gun control laws.
    Liberals favor strict gun control laws.
    Therefore Liberals are [like] Nazis.

  • The Nazis relied on a strong military.
    Conservatives favor a strong military.
    Therefore Conservatives are [like] Nazis.

Loadmaster (talk) 22:21, 16 March 2011 (UTC)Reply[reply]

Removed the anti-religious and fallacious example, twice[edit]

  • Does not add to the article
  • One was itself fallacious (Fallacy fallacy)
  • The second was an example of a different type of fallacy
  • Was unnecessarily political in nature

The provided reference was a quote of a quote of an alleged quote taken out of an out of print reference source and heavily butchered with the use of several ellipses. Please use first-hand sources in any future references you provide, preferably ones that can be followed up on and verified.

Pointing out that a particular creed or worldview has apologists from half a century ago who sometimes make fallacious statements seems like something more suitable for a history article. If you would like to enrich Wikipedia by starting an article about the history and evolution of Baptist doctrine and theology, that would be a a useful contribution. Political axe grinding does not, however, belong in a scientific reference article.

- DevAudio (talkcontribs) 17:50, 1 September 2011 (UTC)Reply[reply]

And again! --Hu5k3rDu (talk) 20:19, 24 March 2018 (UTC)Reply[reply]

Removing example that isn't affirming the consequent.[edit]

Here is the text:

The following is a more subtle version of the fallacy embedded into conversation.
A: All Republicans are against gun control.
B: That's not true. My uncle's against gun control and he's not a Republican.
B attempts to falsify A's conditional statement ("if Republican then against gun control") by providing evidence he believes would contradict ::its implication. However, B's example of his uncle does not contradict A's statement, which says nothing about non-Republicans. What would be ::needed to disprove A's assertion are examples of Republicans who support gun control.

This is not an example of affirming the consequent, as it does not draw the consequent as the conclusion. It is a similar mistake (forming the wrong contradictory of a generalization), but as it is not affirming the consequent, it does not belong in the article. 271828182 (talk) 20:34, 6 May 2012 (UTC)Reply[reply]

A's statement: if Republican then against gun control.
B's statement: A, your statement isn't true. "if against gun control then Republican" does not hold in the case of my uncle.
A: That's not what I'm saying. You're affirming the consequent.--Louiedog (talk) 18:54, 7 May 2012 (UTC)Reply[reply]

He's not affirming the consequent, as he is not concluding "therefore, my uncle is a Republican." 271828182 (talk) 22:36, 7 May 2012 (UTC)Reply[reply]

He's affirming the consequent in order to proclaim that the original implication is not true.--Louiedog (talk) 18:58, 8 May 2012 (UTC)Reply[reply]


This page was tagged as stub in November 2008. Please help expanding this page. -- Magioladitis (talk) 17:02, 9 August 2012 (UTC)Reply[reply]

Expanding the article is a good idea, but I don't see why the date parameter needs to be removed in the meantime, so I have restored it. The parameter is not broken, Mediawiki allows any parameter to be used on any template without ill effects. — Carl (CBM · talk) 17:37, 9 August 2012 (UTC)Reply[reply]

Second Amendment example[edit]

There is an edit war beginning concerning using the Second Amendment, and a historic judgement based on a flawed logical interpretation of it, as an example of the logical fallacy. The text in question is:

The Second Amendment to the United States Constitution, which secures a right to bear arms, is occasionally argued against with an "affirming the consequent" fallacy:

A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed.
The people have a right to keep and bear arms that shall not be infringed.
Therefore, they must be in a militia necessary to the security of a free state.

Because the right to bear arms is written as a tautology, there is no requirement that it only applies in the case of a militia due to the fact that the militia is written as a sufficiency conditional to the right to bear arms. Justice McReynolds committed this fallacy in the Supreme Court case United States v. Miller when he stated that weapons such as sawed-off shotguns were illegal because they could not be used in militias.

McReynolds stated "In the absence of any evidence tending to show that possession or use of a 'shotgun having a barrel of less than eighteen inches in length' at this time has some reasonable relationship to the preservation or efficiency of a well regulated militia, we cannot say that the Second Amendment guarantees the right to keep and bear such an instrument,"307 U.S. 174 (1939) thereby committing the fallacy, and showing the ease at which this fallacy can be overlooked with an argument that is not easily phrased in the standard "If A, then B" form.

My personal opinion is that there is no "if" in the first ("militia") clause, so perhaps it is not a good example of the fallacy. On the other hand, the judge did commit a logical fallacy in his ruling. Discussion, please? — Loadmaster (talk) 16:42, 19 April 2013 (UTC)Reply[reply]

The sentence "A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed" is not a conditional claim. It is an argument:
Premise: A well regulated militia is necessary for the security of a free state.
Conclusion: The right of the people to keep and bear arms shall not be infringed.
N.b. that the offered "example" of affirming the consequent changes the supposed antecedent claim by adding "they must be in a," which is neither stated nor implied in the text of the Second Amendment or in US v Miller. While Justice McReynolds may be mistaken in his interpretation of the Second Amendment, he is not affirming the consequent. This "example" is not to be found in what McReynolds says; it is a strawman. 271828182 (talk) 03:46, 22 April 2013 (UTC)Reply[reply]
U.S. v. Miller decided that a sawed off shotgun may not be owned because it is a weapon that would not normally be used in a "well-regulated militia" - therefore he stated that a "well-regulated militia" is a necessity conditional for "right to bear arms." He is expounding on the idea that part of allowing the populace to be armed is to allow a militia to form as formulated in the Federalist Papers - both given that there was no standing army and in case of government misuse of a standing army (I forget which specific Federalist this was). Regardless, the focus is primarily on the structure of the 2nd itself. There is no guarantee in language that necessity and sufficiency conditionals will be perfectly phrased in an "if - then" format. I could say "when it rains outside the sidewalk gets wet" and it would still be affirming the consequent to state that the sidewalk is wet and therefore it is raining. I could say "One can take antibiotics to cure an infection" but to say "an infection has been cured, therefore they took antibiotics" is affirming the consequent. This argument is very rationally extended to the 2nd Amendment which is clearly in the "The truth of A (a well regulated militia necessary for a free state) necessitates B (the right to bear arms shall not be infringed). To say B is true (you have a right to own guns) but A must also be true, (you can only have guns that aren't sawed off shotguns because those aren't used in a militia), is affirming the consequent. Bloomingdedalus (talk) 22:43, 7 May 2013 (UTC)Reply[reply]
I am well aware that a conditional may be phrased without the words "if" or "then." But a sentence must have an equivalent truth-value to an "if-then" sentence to be a conditional, and the Second Amendment is not such a sentence. It is not posing a hypothetical (as your rain and antibiotics examples do); it asserts both claims, as a premise and its conclusion. Furthermore, even were it a conditional as you describe, to affirm the consequent, McReynolds (or whoever is alleged to have produced this conspicuously unsourced "example") would have to infer as follows:
If a well regulated militia is necessary for the security of a free state, then the right of the people to bear arms shall not be infringed.
The right of the people to bear arms shall not be infringed.
Therefore, a well regulated militia is necessary for the security of a free state.
But that is manifestly not the conclusion at issue ("the people can only have guns used in a militia"). Again, McReynolds may be mistaken about that, but he does not affirm the consequent to derive it. There simply is no source for this alleged example. 271828182 (talk) 08:06, 8 May 2013 (UTC)Reply[reply]

Assessment comment[edit]

The comment(s) below were originally left at Talk:Affirming the consequent/Comments, and are posted here for posterity. Following several discussions in past years, these subpages are now deprecated. The comments may be irrelevant or outdated; if so, please feel free to remove this section.

The article stub could benefit from having cited references. There seems to be disagreement as to the correctness of the definition at the center of this dictionary article. Hotfeba 05:43, 20 July 2007 (UTC)Reply[reply]

Last edited at 05:43, 20 July 2007 (UTC). Substituted at 19:44, 1 May 2016 (UTC)

Bad example in the lede[edit]

Formally the "rain" example is correct. However the real-life reasoning about rain is not fallacious per se. In fact, just the opposite: it is an example of a reasonable deduction. When I look out of the window and I see the wet driveway, I am not going to conclude there must have been a stampede across my yard and now it is covered in cattle piss. Rain is the overwhelmingly most common reason of my driveway becoming wet, therefore my reasoning is sound, and not at all fallacious.

The "Fort Knox" is not a good example for the lede either: it is an obvious nonsense. A good example must be the one where a person indeed may fall into a trap of fallacious reasoning.

See also the first ref (from Fallacy Files). Staszek Lem (talk) 19:48, 30 August 2018 (UTC)Reply[reply]

Affirming the antecedent is not, in fact, affirming the consequent[edit]

Why should affirming the antecedent given the consequent be called affirming the consequent? "Affirming the consequent" should mean the fallacy of saying "(A -> B) => B", not of saying "(A -> B) => (B -> A)". Philgoetz (talk) 02:15, 18 March 2019 (UTC)Reply[reply]

No. This is the formal structure of the fallacy:

Major Premisse: if A then B

Minor Premisse: B

Conclusion: Therefore A.

Simple mnemonic: If the sun shines, I am happy. In fact, I am happy. Therefore the sun shines. But other stuff might make me happy even when the sun is not shining. Sunshine is sufficient, but not necessary. 2A01:CB0C:CD:D800:4CB0:33EA:1F64:4B4 (talk) 14:13, 26 March 2022 (UTC)Reply[reply]