Talk:Denying the antecedent

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is a double page with Non sequitur (logic), i suggest it should redirect to that page.

Old, but I'll bite. This is a specific type of non sequitur, but this page (and related pages on logical fallacies) go into more depth than non sequitur (logic) should. —Cuiviénen 20:10, 11 February 2007 (UTC)

in the definition given here, if P is a necessary condition for Q, is it still a fallacy? Somaticvibe 21:39, 19 February 2007 (UTC)

no. --YbborT 22:11, 19 February 2007 (UTC)
  • "if P is a necessary condition for Q, is it still a fallacy". No, because then If P, then Q becomes If, and only if, P, then Q, which also implies If Q, then P. Then, by the modus tollens, not P, therefore not Q. — Isilanes 10:58, 22 February 2007 (UTC)

[edit] Turing Quote

It looks to me as if this quote was intended to mean "if and only if," and thus is not a relevant example to this section. — Twey 02:56, 21 October 2007 (UTC)

Read the essay. Turing explicitly intends the argument to be invalid: he erects it only to knock it down. 271828182 03:13, 21 October 2007 (UTC)
Yes, but quoting the article out of context leads to confusion here, because I think most people would agree with Turing's fake argument because it rests upon the common notion people hold that "people are machines if and only if they follow a set of rules to live by", as Twey pointed out. Obviously Turing does not agree with that proposition, but that is not obvious reading the wiki article. —Preceding unsigned comment added by 144.80.57.194 (talk) 05:58, 15 February 2008 (UTC)
Misreading a conditional as a biconditional, however common it may be, is simply an error. Indeed, it is a major reason why denying the antecedent is a widespread fallacy. That the error is not obvious is the point, as the text introducing the example explicitly observes. 271828182 (talk) 03:42, 16 February 2008 (UTC)
Turing begins the essay by defining machines, as discrete machines; machines for Turing are by definition ones which only follow sets of rules. If you had an 'object' which did not follow rules, I don't think Turing would count it as a 'machine'. There is an implied premise which makes the argument valid. That being that machines are things which follow rules. March 4 2008.
Yes. Turing discusses just such an additional premise, and goes on to argue against it, in the passage cited. 271828182 (talk) 17:26, 4 March 2008 (UTC)

[edit] To His Coy Mistress

  • Is this link appropriate (in the "see also" section)? The editor who included it says that the poem is a good example of denying the antecedent... I don't really see it so clear. Opinions? — Isilanes 20:37, 4 March 2007 (UTC)
    • "Had we but world enough and time, this coyness, lady, were no crime" = "If we had time, then it'd be okay to be coy." (If A, then B)
    • "But at my back I always hear time's winged chariot drawing near" = "We don't have time." (Not A)
    • "Now let us sport while we may" = "Coyness is a crime, or, Let's Get It On" (Not B)

271828182 00:25, 5 March 2007 (UTC)

Very good! On other hand, the argument does seem valid. edward (buckner) 15:55, 5 March 2007 (UTC)

I think it is valid, but as an instance of the principle of sufficient reason. If it were the case that A, then that would be a reason for B. But it is not the case that A. Thus (assuming a supressed premiss to the effect that there is no other reason for being B), the principle of sufficient reason implies not-B. But hang on. There has to be a sufficient reason for not B, doesn't there? Let me think about that. edward (buckner) 15:58, 5 March 2007 (UTC)

Or another interpretation could be:

If we wait, there is time (to wait) But there is no time Therefore we don't wait

  • I must admit there is a temptation to rescue the poem by reading the first line as a biconditional. But to stick to what Marvell wrote, we must conclude that not only is the poet's motive impure, but so too is his logic. 271828182 01:03, 6 March 2007 (UTC)
  • Forgive my English literature ignorance, but... is this poem so notable and well know that deserves mention? Is this the best known and most clear example of DA in English literature? A Wikipedia reader following the link will have her doubts on the DA subject decreased, or increased, by reading this poem? Regarding the explanation of the DA in the poem above, I have my doubts. I understand the following: "Had we but world enough and time, this coyness, lady, were no crime", not as "If we had time, then it'd be okay to be coy.", but rather, "Only if we had time would coyness be OK", or, in other words, the "if" is an "if and only if", so the reasoning of the poet is correct by the modus tollens. The poet means that the only reason for being coy would be to have plenty of time in their hands (which is not the case), not that it would be one of many reasons. — Isilanes 08:31, 6 March 2007 (UTC)
I don't know if best known as DA, but, trust me, this is a very well poem. I found it quite amusing and a good example. edward (buckner) 12:56, 6 March 2007 (UTC)
On the reading of it, yes, that's possible. But the reading of 'but' in 'if we had but an X' usually implies sufficient condition, not necessary. Thus a fallacy. edward (buckner) 12:59, 6 March 2007 (UTC)
Well, it is a very famous poem. Is it the clearest example of DA? No, but non-textbook examples of logical fallacies are seldom obvious. ("Had we but" is not equivalent to iff: do a Google search on an equivalent phrase such as "had I but" to see many examples of usage where the iff reading makes no sense.) Will a Wikipedia reader grasp that the poem is an example of DA? Maybe, maybe not. Will they read some great poetry and see how fallacious inferences can go generally unnoticed? I would hope so. 271828182 17:36, 6 March 2007 (UTC)
"Had I but a little slice of bread, I would not be dying of hunger!" "Ah. So a piece of ham wouldn't have helped."
  • Great! I just wanted to rest assured that its addition would be positive, as it seems it is. — Isilanes 18:08, 6 March 2007 (UTC)

Well, didn't see this, weird and amusing, but shouldn't this be cross-linked in the actual article on the poem? Obscurans 01:41, 11 July 2007 (UTC)

[edit] well...

If I have a boyfriend, I will go to the dance tomorrow.

I don't have a boyfriend.

But I can still go to the dance tomorrow. —Preceding unsigned comment added by 68.101.123.219 (talk) 19:37, 17 February 2008 (UTC)

Actually this might be a more relevant and understandable example for the article. 118.90.72.183 (talk) 15:00, 7 June 2008 (UTC)


Actually, in DA, it should actually be:

"If I have a boyfriend, I will go to the dance tomorrow.

I don't have a boyfriend.

So I can't go to the dance tomorrow."

64.231.120.29 (talk) 02:19, 14 June 2008 (UTC)