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I ask you to go to the store and buy some eggs. If you get some eggs, I will pay you back. You bring the eggs back. But I tell you "oh they aren't of brand X, so I won't pay you back".

How can you describe a situation where someone withholds information like this, whether knowing or unknowing?

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It's not about withholding information. Eggs of any kind are eggs, and the condition was that if you buy some, of whatever kind, then you will be paid. What's said at the end doesn't matter; either the promise will be broken or you will get paid. –  Hunan Rostomyan May 19 at 17:11
    
I see. So the statement is actually about the promise, and not about the eggs at all. –  user6786 May 19 at 19:00
    
That's just my opinion. Other, more interesting alternatives have been offered below. –  Hunan Rostomyan May 19 at 20:20

4 Answers 4

up vote 5 down vote accepted

If this were an argument, and not a request, we would call it moving the goalposts, which is where the original conditions are met, but additional ones are subsequently added.

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perfect thank you –  user6786 May 19 at 21:40
    
Right, IF it were an argument. But there is a logical problem here that has to do with just the asserted statements, without treating the situation as analogous to argumentation. –  ChristopherE May 21 at 14:44

I think we cannot call it a Logical fallacy.

It look like :

Denying the antecedent – the consequent in an indicative conditional is claimed to be false because the antecedent is false; if A, then B; not A, therefore not B

with A := "you buy some eggs for me" and B := "I will pay you back"

but in addition we have an "hidden" premise C := "eggs must be of brand X".

In any case, from

( A ∧ C ) → B and ( A ∧ ¬C )

does not follow :

¬B.

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Cool, thank you very much –  user6786 May 19 at 17:08

Default logic might be of interest. It is non-monotonic, so weakening does not always hold. Usually if A implies B, then A and C imply B, for any C. But consider the famous example: Tweety is a bird, so by default "Tweety can fly" is true. But then we find out Tweety is actually a penguin. Under this new assumption, "Tweety can fly" is false.

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This could be understood simply as a contradiction, if the first speaker is saying that she will repay all egg purchases, and then later that she will not repay all egg purchases.

Alternately, this could be understood as an example of “ad hoc” revision: a generalization was stated in such a way that it appeared to be intended as always true, but then it was later modified to address a particular case. Perhaps the first speaker didn't actually expect that anyone would buy eggs, for instance, and faced with someone calling her bluff and actually buying the eggs, she backs off from it by suggesting that all along the generalization she intended to express was actually "If you get some eggs I will pay you back unless they are brand X." One wonders whether if you returned with different eggs that she didn't want to pay for, whether she might then modify the generalization to be "If you get some eggs I will pay you back unless they are brand X or from Store Y." (And perhaps she might claim that that was the actual generalization all along, but it's clear that it was not.) This kind of modification is the hallmark of "ad hoc" reasoning. It means in Latin, "for this," because a change is made in a position to address a particular case. The change is made "for this case." Often that case is an exception to the original generalization, like "I love everyone! ... oh, right, except him ... or him ..." Here it is a way of rejecting an instance of the generalization rather than an exception to it.

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