Is this an example of the fallacy of affirming the consequent?



Would this be an example of the "fallacy of affirming the consequent"?

If suspect X did the deed, then we would find evidence A, B, C and D. And, yes, look, here is evidence A, B, C and D. So suspect X is the felon.


Posted 2017-10-31T14:32:28.890

Reputation: 71


Correct. See Affirming the consequent.

– Mauro ALLEGRANZA – 2017-10-31T14:45:23.403

@Paul Ross : There are two answers to your affirming the consequent question. I realise you asked the question a while back but there are responses you might be interested to consider. All the best : Geoffrey Thomas – Geoffrey Thomas – 2017-11-21T22:56:07.577

@GeoffreyThomas Paul Ross is not the one asking the question. He merely edited the question. – virmaior – 2017-11-22T01:35:11.800



The fallacy of affirming the consequent can be set out formally :

p → q


Therefore p

We cannot validly infer the antecedent 'p' from the conditional schema and the affirmation of the consequent 'q'.

The conditional could be true ('If p then q') and the consequent, 'q', true. Yet 'p' could be false. This would violate the rule that a falsehood cannot be validly inferred from truth(s) :

If p then q (true) q (true)

p (false)

In an obvious example, take :

If it is raining, then the pavements are wet (true) The pavements are wet (true)

Therefore it is raining (false)

'If it is raining, then the pavements are wet' - true conditional. 'The pavements are wet' - true consequent. They really are wet. But it isn't raining - false antecedent. The pavements are wet because a water main has burst.

Hope this helps, makes things a bit clearer : it shows that your question was spot on. Your logical instincts are sound.

Geoffrey Thomas

Posted 2017-10-31T14:32:28.890

Reputation: 34 276


Wikipedia describes affirming the consequent in the following way:

Affirming the consequent, sometimes called converse error, fallacy of the converse, or confusion of necessity and sufficiency, is a formal fallacy of taking a true conditional statement (e.g., "If the lamp were broken, then the room would be dark,") and invalidly inferring its converse ("The room is dark, so the lamp is broken,") even though the converse may not be true. This arises when a consequent ("the room would be dark") has one or more other antecedents (for example, "the lamp is not plugged in" or "the lamp is in working order, but is switched off").

Note how this fallacy is described as the "confusion of necessity and sufficiency".

It is not necessary that the lamp be broken if the room is dark. The lamp being broken is only a sufficient condition for the room to be dark. There may be other explanations besides a broken lamp which would also be sufficient to darken the room.

However, if we know that the lamp actually is broken and we also know that this broken lamp is a sufficient condition for the room being dark, then the room would be necessarily dark because a sufficient condition for the room being dark has been fulfilled. But in this case all we know is that the room is dark.

They also explain why this fallacy is common.

Converse errors are common in everyday thinking and communication and can result from, among other causes, communication issues, misconceptions about logic, and failure to consider other causes.

Consider the example:

If suspect X did the deed, then we would find evidence A, B, C and D. And, yes, look, here is evidence A, B, C and D. So suspect X is the felon.

This is an example of the fallacy, because we are assuming our knowledge of the evidence and that X being the felon would be a sufficient condition for the evidence appearing as it does means that X is necessarily the felon.

That doesn't mean X is free to go. X may in fact be the felon, but we don't know for sure given what we know of the evidence. The fallacy is in claiming that X must be the felon, that X is a necessary condition for the evidence to exist rather than only a sufficient condition.


"Affirming the consequent", Wikipedia

"Necessity and sufficiency", Wikipedia

Frank Hubeny

Posted 2017-10-31T14:32:28.890

Reputation: 18 742


Yes this is a fallacy because of the fact we can have instances of true premises and a false conclusion.

If the Earth is the third planet from the Sun, then you are Santa Claus.

You could put whatever you desire after the word THEN.

Some people will find instances where everything checks out factually and they think the resoning works because it works right then and there. Thus this resoning is at best shakey. Sometimes it yields correct conclusions and sometimes it does not yield correct conclusions.


Posted 2017-10-31T14:32:28.890

Reputation: 409