Affirmation of the consequent

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Joe
 
Reply Sat 1 Nov, 2008 12:32 pm
I read this as one of many fallacies that are used to sidetrack what is logical. Affirmation of the consequent states that, A implies B, B is true, therefore A is true.

So A implies B. But how can B still be true when your relating it to A which has not been stated as true, until it is compared to something that is?

I know its simple but it seems that if this is one of the most simple concepts in logic, then I have to make changes to everything I believe is true.
 
kennethamy
 
Reply Mon 3 Nov, 2008 08:38 am
@Joe,
Joe wrote:
I read this as one of many fallacies that are used to sidetrack what is logical. Affirmation of the consequent states that, A implies B, B is true, therefore A is true.

So A implies B. But how can B still be true when your relating it to A which has not been stated as true, until it is compared to something that is?

I know its simple but it seems that if this is one of the most simple concepts in logic, then I have to make changes to everything I believe is true.



The fallacy is to suppose that because A > B, and because B is true, that A is true. For instance, if it is true that if something is a dog then it is a mammal, and if it is true that something is a mammal, it would be fallacious to conclude that something was a dog.

A is not stated as true, and only B is stated as true (in the second premise) but it is not stated as true because of A.

Essentially, the fallacy consists in thinking that because A is a sufficient condition of B, that B is also a sufficient condition of A. In fact, B is a necessary condition (but not sufficient condition of A) So, in my example above, although being a dog is a sufficient condition of being a mammal,it does that follow that being a mammal is a sufficient condition of being a dog. And, in effect, that is what the fallacious argument says: because A is a sufficient condition of B, B is a sufficient condition of A. And that's wrong.
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ogre
 
Reply Fri 16 Jan, 2009 08:12 am
@Joe,
It also depends on the logical operator you apply to the condition. There is as far as I know 3 logical operators... And, Or, Not. Each will give you a different answer. Without sufficient data you cannot make an assumption.
 
 

 
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