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What are common examples of affirming the consequent reasoning you see in everyday life?

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    See Affirming the consequent : "The room is dark, so the lamp is broken". – Mauro ALLEGRANZA Jan 24 at 15:50
  • Seems like for this to be affirming the consequent it would need to be: If the lamp is broken then room is dark. The room is dark. Therefore, the lamp is broken. Am I right? And, of course, this is a fallacy. That's what I'm interested in. Everyday examples where people use Affirming the Consequent fallacy and get tripped up. – oaktrees Jan 24 at 16:03
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Affirming the consequent is not a flaw in thinking: it's a rhetorical tool, like redirection or enthymeme, that people use from time to time in order to convince others of their position.

Formally speaking, affirming the consequent is a true description of the first two clauses of the Declaration of Independence:

In Congress, July 4, 1776.

The unanimous Declaration of the thirteen united States of America

Before these words became the consensus of Congress, no such thing as "the united States of America" existed. The writers affirmed the consequent, that they were no longer obligated to the throne.

Affirming the consequent is also used during antagonism, as when somebody says, "You're stupid." Will this statement be followed with evidence that "you're stupid" that proves that "you're stupid"? Maybe... but the opening line, "you're stupid" affirms the consequent and attempts to re-frame "your" idea of what kind of person you are even before the evidence comes out.

And if you believe that you're stupid, then the other person is not only right, they are your best friend because they told you something true about yourself that you were too stupid to see. So of course whatever else they are going to say is a good idea, too.

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Many examples of this fallacy I can think of are committed when trying to reason about diagnosis and cause of diseases.

When someone has AIDS, people jump to the conclusion that he had unprotected sex or he is on drugs (same for smoking...etc).

It is obvious that these conclusions are generally built on an affirming the consequent fallacy. Because if we were to translate it into propositional logic, we would have

  • Premise 1 : If SMOKING then HEALTH-PROBLEMS
  • Premise 2 : HEALTH-PROBLEMS
  • Conclusion : Therefore, SMOKING

As you can see, the first premise is true (assuming that HEALTH-PROBLEMS here means anything that is linked to smoking, even coughing).

It is not the case that anyone who is coughing, or having lung disease is necessarily smoking, and yet people assume that one is smoking if one has lung cancer, or conclude that one has had unprotected sex if one has AIDS.

So, from this perspective, it is an affirming the consequent.

Note : Of course we cannot consider it an affirming the consequent if the argument were of this form :

  • Premise 1 : If HEALTH-PROBLEMS then SMOKING
  • Premise 2 : HEALTH-PROBLEMS
  • Conclusion : Therefore, SMOKING

If someone concludes that one is smoking following the previous form of reasoning, then it is not an affirming the consequent, but rather a valid Modus Ponens.

Although it is valid, in this case the first premise is false, since HEALTH-PROBLEMS does not necessarily imply SMOKING.

So, this type of reasoning about disease problems is generally built on:

  • An affirming the consequent fallacy (with the conditional being true)
  • Or, a valid Modus Ponens (with the conditional being false).

Most of these arguments are either valid with a false premise, or invalid with a true premise.

Except for the cases where a disease is caused by one and only one factor (which is very rare).

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Affirming the consequent is a common reason for jumping to a conclusion by ignoring alternative explanations. Since ignoring alternatives is part of the logical flaw, this is often also black-and-white thinking or a suppressed premise that eliminates options.

But in this case what makes the statement seem credible, excusing the more serious fallacy to which one might not otherwise fall prey, is that the converse of what is being said is logical.

If you didn't support Hillary Clinton, you must be a Republican

Well, there was a Libertarian and a Green Party candidate in that election. So this is not logically certain. Republicans usually don't vote for establishment Democrats, but other people also don't.

If you are well-informed, you must read a lot.

Well, there are other sources of information. Maybe this person listens to a lot of taped lectures. Reading is a way of being informed, but only one.

If a woman makes a pass at you and you ignore it, you must be gay.

Maybe I am married.

  • Yeah there's a lot of crossover between converse error and false dilemma. Some things are such bad arguments.. it's hard to know which fallacy is most prominent. The Clinton argument above being a great example. My favourite false dilemma is Trump's you're either with us.. or against us. – Richard Jan 25 at 0:37
  • @Richard Don't give that man any credit -- you mean Jesus's! That is a Bible quote, both ways around. "Who is not with me is against me" is in Matthew and "Who is not against us is with us" is in Mark. But yeah, I generally see the more formal error as a failed check against the less formal error -- you had one extra opportunity to not be wrong because of the form of the proposition, and you missed it. After all, every fallacy is already 'non-sequitur', so there is never just one fallacy at play. – jobermark Jan 25 at 1:05
  • I wasn't aware that it was a biblical quote. It's a ludicrous fallacy whoever said it.. but amusingly it rather excuses Trump if he was quoting. – Richard Jan 25 at 3:03

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