November 25, 2005

Logical fallacies in real life: affirming the consequent

The logical fallacy called affirming the consequent has the following form:
  1. (Hypothesis) If P, then Q
  2. (Hypothesis) Q
  3. (Conclusion) P

Here's a simple argument to illustrate the fallacy:
  1. If I go on a 20-mile bike ride, I will be tired.
  2. I am tired.
  3. Therefore, I went on a 20-mile bike ride.

The argument is fallacious because P need not be necessary for Q to come about.

Here's a real life illustration. P would be something like 'the Left supports the Enemy' while Q might be 'the Left is critical of the war' or 'the Enemy wins the war'.

This interview cannot be read without coming to the conclusion that the Left was aiding and abetting the enemy back then, and they are doing it again today. Period.

For lots of examples of logical fallacies (not to mention sexism, racism, and a nasty temper), read this one, too.


The Exile said...

Logical fallacy? Affirming the consequent? Your "intellectualism" really intimidates me (smirk).

You seem to have forgotten the point that P might actually equal Q.

Here's a simple argument to illustrate your omission.

- If I go on a 20-mile bike ride, I will be tired.
- I am tired
- I went on a 20-mile bike ride, therefore I'm tired.

Your pseudo-intellectualism does not become you.

Do you have any idea how silly you are?

Noumena said...

I don't follow your example. It should be analysed as follows, if we're looking at its logical form:
1. If I go on a 20-mile bike ride, I will be tired.
2. I am tired.
3. I went on a 20-mile bike ride.
4. Hence, I'm tired.

Then the first and third hypothesis are redundant to show the conclusion. Is that your point? If so, then I fail to see where two propositions are being set 'equal' to each other, or how this illustrates some omission I made in my discussion in the post. Could you elaborate a bit?

Incidentally, I'm not a pseudo-intellectual. If you check my profile, you'll see that I'm a bona fide intellectual.

Unknown said...

Wow, Noumena, you're right. I've never seen such specious claptrap. But in all fairness, most blogs (including my own) are basicaly just logic-free rant zones. Beating up on a blogger for logical fallacies hardly seems sporting.

Still, I've always been a sucker for logical fallacies. I mean, when you write them out formally, they look so obvious that you wonder how anyone could possibly be stupid enough to commit them. And then you look around and you realize that everyone does, all the time.

And as you point out, The Exile's post is notable more for its racism and sexism than its illogic.

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