Tuesday, March 31, 2009

Confirming the Consequent

This is a relatively simple formal deductive logical fallacy. There is a valid form of propositional arguement called modus ponens. It takes the form "If p then q", and "p", therefore "q". This is logically valid, and if one precedes from true premesis, one will reach a true conclusion. An example would be "If it is raining on my front yard, the pavement of my sidewalk will be wet", "It's raining on my front yard" therefore "My sidewalk is wet".

However, there is a desire many times to make the argument "If p then q" and "q" therefore "p". This is the fallacy of confirming the antecedant. The truth of q in no way supports or denies the truth of p. If I say "If it's raining on my front yard, my sidewalk will be wet" then observe "my sidewalk is wet", I cannot assert "it is raining". My sidewalk may have gotten wet because someone sprayed it with a hose, or the snow is melting, or there's a big puddle in the street that was splashed onto the sidewalk.

However, confirming the consequent is a fallacy perpetrated commonly in the world. A more complex example would be to say "If Obama deceived the American people about his eligibilty to become president then he would run for office and win." and then to say "Obama ran for office and won." and then conclude "Obama must have deceived the American people about his eligibility."

Googlebombing for a cause: www.minnesotangos.org 


Anonymous said...

You have to express more your opinion to attract more readers, because just a video or plain text without any personal approach is not that valuable. But it is just form my point of view

Anonymous said...

Conseils tres interessants. A quand la suite?

Leo Bushkin said...

There are cases where affirming the consequent is not a logical fallacy. When the argument is structured: If and only if P the Q, Q therefore P. For example: "If and only if the cake is chocolate will Henry eat a cake. Henry is eating a cake, therefore the cake is chocolate."

Lord Carnifex said...

Leo Bushkin, thanks for the comment.

You're exactly correct. If we have an argument in the form of "If and only if P, then Q" and we assert Q, then we can logically conclude P. BTW: I like to abbreviate the somewhat cumbersome phrase 'if and only if' as 'iff', pronounced 'ifef'

In a bastard ASCII mockery of formal notation, we'll call your argument:

1) P <=> Q
2) Q
ergo 3) P

This is perfectly valid. This is also not quite confirming the consequent. Confirming the consequent is a fallacy when used on the somewhat simpler conditional 'if' rather than iff. In our bastard ASCII, we notate confirming the consequent as:

1) P => Q
2) Q
ergo 3)P

which is not a valid argument form.

Which reminds me, to continue my discussion of logical fallacies and errors of reasoning. Your comment could well use an additional explanation of necessary and sufficient conditions.

Anonymous said...

why not:)