Moore’s Proof of an External World. Certainty and Error. One thing Russell seems right about is that we don’t need certainty in order to know something. In fact, even Descartes grants this. Certainty and Error. The Certainty Argument
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One thing Russell seems right about is that we don’t need certaintyin order to know something.
In fact, even Descartes grants this.
The Certainty Argument
No one is ever certain of anything about the external world.
If S knows that p then S is absolutely certain that p.
Therefore, no one knows anything about the external world.
Possibility of Error Argument
For any belief, p, S has about the external world S could be mistaken.
If a belief could be mistaken, then it is not a case of knowledge
Therefore, S does not know anything about the external world.
It seems very easy to respond to the skeptic if we can just deny the second premise in these arguments.
Unfortunately there is another skeptical argument that does not rely on such implausible premises and has the same radical conclusion.
Consider the following principle:
Transmissibility: If I know p is true, and I also know that p entails q, then I can know that q is true.
Is this principle true?
It sure seems like it.
If I know p, and know that p entails q, then it seems I could simply draw the inference in order to come to know that q!
Unfortunately, if this principle is true, we can generate a powerful skeptical argument that does not depend on the requirement that knowledge be absolutely certain.
The Transmissibility Argument (Table)
I can’t know that I am not in the Matrix.
If there is a table here, then I am not in the Matrix.
I know that 2 is true.
If I know that there is a table here, and I also know that this is incompatible with being in the Matrix, then I can know I’m not in the Matrix.
Therefore, I can’t know there is a table here.
There is nothing special about the table. I could run the same reasoning for any belief about the external world.
One can immediately see that if the argument is sound, then we don’t know anything about the external world.
The Transmissibility Argument (Hands)
I can’t know that I am not a BIV.
If I have hands, then I am not a BIV.
I know that 2 is true.
If I know that I have hands, and I also know that this is incompatible with being a BIV, then I can know I’m not a BIV.
Therefore, I can’t know that I have hands.
The argument is valid.
Unlike the Certainty and Error arguments, none of the premises are obviously false.
This is the puzzle that really requires an answer and it has stumped philosophers (and everyone else) for hundreds of years.
Finish Bouwsma. Start reading Thomson’s “A Defense of Abortion” (59-79)
Jenna’s office hours today are cancelled. They will be rescheduled for next week.
Moore claims that the proof is “perfectly rigorous” and that “it is perhaps impossible to give a better more rigorous proof of anything whatever.” (143)
Three conditions for a rigorous proof:
We ordinarily accept proofs of this kind:
I don’t believe my roommate when he tells me there is beer in the fridge.
He can properly prove me to be mistaken by going to the fridge and pointing out a couple of beers.
Similarly if we doubt that there are external objects, all that is needed is for us to produce one!
Moore concedes that he didn’t prove that he has hands.
However, as we already know proof and certainty are not required for knowledge!
“I can know things which I cannot prove; and among things which I certainly did know, even if (as I think) I could not prove them, were the premises of my two proofs.” (105)
But Moore seems to beg the question against the skeptic.
One begs the question against if one assumes the falsehood of an argument’s conclusion in one’s premises.
The skeptic gives an argument for the conclusion that Moore doesn’t know that he has hands.
Moore simply assumes that this is false without any argument!
Moore thinks that if he is begging the question against the skeptic, the skeptic equallybegs the question against him!
Skeptical Argument (condensed)
Skeptical Argument (condensed)
Moore thinks that all the premises of both arguments are very plausible.
However, the one he is least likely to give up on is the claim that he knows he has hands (and other similar claims).
Ambitious Anti-Skeptical Project: Refute a skeptic on his own terms. Show to the satisfaction of the skeptic that one knows about the external world.
Modest Anti-Skeptical Project: Establish to our own satisfaction that we know things about the external world, without contradicting obvious facts about perception and evidence.
Does Moore succeed in convincing us by our own standardsthat we know things about the external world?
For all of his cleverness, few people are convinced by Moore’s argument on its own.
Moore has shown that the following three claims are inconsistent:
Transmissibility: If I know p, and know that p entails q, then I can know q.
I can’t know that I am not in the Matrix/dreaming/a BIV/etc.
I know that I have hands.
He may even be right that most of us won’t give up on #3 as the skeptic demands.
Without an answer to this puzzle, it is hard to shake the feeling that we are just being dogmatic about our supposed knowledge.
If we have no rational reason to prefer #3 to the other two propositions, or know which of #1 or #2 are false shouldn’t we suspend judgment?