Logical Fallacy: Affirming the consequent

Affirming the consequent has the formal structure

  1. If P, then Q.
  2. Q.
  3. Therefore, P.

One can imagine two clouds, one marked P and the other marked Q. There’s an arrow from P to Q, which is the first premise. But there might be other clouds (R, S, T, etc) that point to Q that we just haven’t bothered identifying, so it’s not necessarily the case that because you’re at P you came from Q.

“A big part of being a conservative is believing in balance budgets.” “Well, I agree that we should have balanced budgets.” “Then you’re a conservative.” Not necessarily, since political positions don’t always have monopolies on preferred policies (say that ten times fast). The classic example of this fallacy is:

  1. If it rains, the streets will be wet.
  2. The streets are wet.
  3. Therefore, it rained.

Unless perhaps a fire hydrant broke. I don’t particularly like the name of this fallacy since it uses fairly technical language (another name for it is the converse error). I might have chosen to call it something like the determinism fallacy, since you’re assuming where you could have come from is where you must have come from.

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