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  1. I cannot be certain of anything. (Assumption.)
  2. I am not certain that I cannot be certain of anything.
  3. By asserting (2), I am certain that I am not certain that I cannot be certain of anything.
  4. I can be certain of something (3), therefore it's not the case that I cannot be certain of anything.

I can't spot my mistake... can you help?

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  • Consider the language E-prime. The problem is that you're making predicates that are mutually incompatible.
    – Marxos
    Commented Jun 13, 2013 at 19:09
  • 2
    It is just an example that radical skepticism is fallacious
    – Michael16
    Commented May 20 at 16:50
  • 1
    The mistake is that all your statements contain the word 'I'.
    – Scott Rowe
    Commented May 21 at 1:10
  • Certainty is not a yes or no switch, it's a spectrum: we can be absolutely certain of something, fairly certain, doubtful, etc. There is no contradiction in asserting "I am fairly certain that I can't be absolutely certain of anything", which in fine boils down to being ready to change your mind about anything if shown sufficient evidence.
    – armand
    Commented May 23 at 0:36

2 Answers 2

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Your assumption (1) is a certainty - you have made it an axiom.

Therefore 2: 'I'm not certain that I cannot be certain of anything', is invalid.

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  • 1
    Does this mean my assumption is false? I can assert that I can be certain of some things? Is this a reasonable dichotomy?
    – user72273
    Commented May 16, 2013 at 2:29
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    Doesn't inconsistency mean that the assumption is not a reasonable assumption?
    – user72273
    Commented May 16, 2013 at 2:52
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    @user72273 you can assume it, but, as you see, it leads to this problem. So it isn't a very good assumption, is it?
    – user2953
    Commented May 16, 2013 at 5:59
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    Premise: All of my thoughts may be deluded and false. I can't be certain that this isn't the case. Therefore, I am certain that I can't be certain that this isn't the case. Therefore, by contradiction, I have a bad assumption. Not all of my thoughts are deluded and false. Is this logical?
    – user72273
    Commented May 16, 2013 at 7:03
  • 3
    No, it is not: Again, according to your original premise, you can never be certain of anything. You are attempting to build certainty from a 'premise' of uncertainty. This cannot work.
    – Vector
    Commented May 16, 2013 at 7:35
0

I would say that the inference from (1) to (2) is valid. The problem I have with the argument is the transition from (2) to (3). I am going to rewrite the argument first:

  1. Assume that it is impossible for me to be certain of the truth of any proposition.
  2. From (1) it follows that I am not certain that it is impossible for me to be certain of anything is true.
  3. From (1) and (2), I am certain that I am not certain that it is impossible for me to be certain of anything. (invalid)

Here is a less logicky example showing how (2) can be true while (3) false.

Suppose I think all hippos are gray is true, but (at first blush) I'm a little hesitant as I think that there might be some red hippos in a far away land. On the basis of this, we would say I am not certain that "all hippos are grey" is true. Does it follow that I am certain that I am not certain that proposition that "all hippos are grey" is true? No, for even if we assume that I am uncertain about whether all hippos are grey, I might not be certain that I'm uncertain. I might say to myself, "ya know? I thought I don't even know if I am uncertain. Maybe I am certain that all hippos are grey.

To make the argument valid. You need another premise that our knowledge of our own epistemic/mental states is always transparent to us.

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