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Affirming the Consequent

(also known as: converse error, fallacy of the consequent, asserting the consequent, affirmation of the consequent)

New Terminology:

Consequent: the propositional component of a conditional proposition whose truth is conditional; or simply put, what comes after the “then” in an “if/then” statement.

Antecedent: the propositional component of a conditional proposition whose truth is the condition for the truth of the consequent; or simply put, what comes after the “if” in an “if/then” statement.

Description: An error in formal logic where if the consequent is said to be true, the antecedent is said to be true, as a result.

Logical Form:

If P then Q.


Therefore, P.

Example #1:

If taxes are lowered, I will have more money to spend.

I have more money to spend.

Therefore, taxes must have been lowered.

Explanation: I could have had more money to spend simply because I gave up crack-cocaine, prostitute solicitation, and baby-seal-clubbing expeditions.

Example #2:

If it’s brown, flush it down.

I flushed it down.

Therefore, it was brown.

Explanation: No!  I did not have to follow the, “if it’s yellow, let it mellow” rule -- in fact, if I did follow that rule I would probably still be single.  The stated rule is simply, “if it’s brown” (the antecedent), then (implied), “flush it down” (the consequent).  From this, we cannot imply that we can ONLY flush it down if it is brown.  That is a mistake -- a logical fallacy.

Exception: None.

Tip: If it’s yellow, flush it down too.


Jevons, W. S. (1872). Elementary lessons in logic: deductive and inductive : with copious questions and examples, and a vocabulary of logical terms. Macmillan.

Registered User Comments

Monday, August 13, 2018 - 12:01:16 AM
Helly Sir,
I have a case in argumentation where some one did criminal things in the past but not being jailed until today. The actor was someone with a high range military position, investigated by a special team. The team revealed the actor proven committed to a crime.
Mr. A committed to a crime, he should have been in the jail. He is not in the jail, so he was not a criminal.
Is this a fallacy?
Thank you

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Bo Bennett, PhD
Monday, August 13, 2018 - 06:55:15 AM
What you actually have is this:

If P then Q.
Not Q.
Therefore, not P.

This is Denying the Antecedent, and is fallacious.

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Tuesday, August 14, 2018 - 12:20:21 AM
@Bo Bennett, PhD: Thank you for your response. I agree with "barking dog" example, it helps me to understand my case.
Thank you.

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Sunday, July 22, 2018 - 10:11:53 PM
Just wanted to say, I'm a student that has gone to SO many websites to achieve understanding with all of my classes, and your website is by far the most entertaining whilst still being super-informative. You have my gratitude.

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Krista Neckles
Monday, July 02, 2018 - 05:51:55 PM
Hello Sir,

There exists the pure hypothetical syllogism where one says:

If p then q
If q then r
Therefore if p then r

Supposedly this is valid. What is invalid is for instance:

If p then q
If r then q
Therefore if p then r.

What fallacy is this invalid form called?


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