Thursday, October 7, 2010

A fallacy in non-deductive reasoning

If we think about the large variation of appearances among donkeys, it is very probable that:

  1. Some donkeys look very much like horses.
Now we run this argument:
  1. D is a donkey that looks very much like a horse. (Existential instantiation on 1)
  2. D looks very much like a horse. (By 2)
  3. D is a horse. (Probabilistic inference from 3)
  4. Some donkey is a horses.
We might make the inference from (3) to (4) more explicit by making use of:
  1. If D looks like a horse, probably D is a horse.
I think this is related to Rowe's argument from evil.

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