http://www.iep.utm.edu/brainvat/#H2
The Brain in a Vat Argument
The Brain in a Vat thought-experiment is most commonly used to illustrate
global or Cartesian skepticism. You are told to imagine the possibility
that at this very moment you are actually a brain hooked up to a sophisticated
computer program that can perfectly simulate experiences of the outside
world. Here is the skeptical argument. If you cannot now be sure that you
are not a brain in a vat, then you cannot rule out the possibility that
all of your beliefs about the external world are false. Or, to put it in
terms of knowledge claims, we can construct the following skeptical argument.
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Let “P” stand for any belief or claim about the external world, say,
that snow is white.
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1. If I know that P, then I know that I am not a brain in a vat
2. I do not know that I am not a brain in a vat
3. Thus, I do not know that P.
以上是節錄一段網站桶中腦的文章
我覺得premise1 是個有問題的句子
premise1: If I know that P, then I know that I am not a brain in a vat
請問大家 "如果我知道P 那麼我不是桶中腦" 這句為甚麼會成立?
我看了版上關於桶中腦的討論 都沒有人質疑Putnam 我覺得很奇怪
※ 引述《aletheia (HERESY)》之銘言：
: 桶中大腦（BIV）。
: 常被用來當作反對外在世界存在的例子。
: 就形上學來說，BIV的確反對形上學的實在論(MR)
: 但以知識論來說，BIV不反對知識論上的實在論(ER)
: 何解？

前提 1 是懷疑論命題的邏輯等值語句，懷疑論命題是：If I do not know that I am not a B.I.V., then Idon't know that P (given any P).將懷疑論命題以異質位換（contraposition）將前後件位置對調並各自加上否定後，便得前提 1。如果你認為前提 1 有問題，那麼其實就是認為懷疑論命題有問題，也就是認為懷疑論的結論是有問題的。另外，前提 1 並不是 Putnam 的主張，所以我也不清楚為什麼你會認為前提 1 值得懷疑就意味著 Putnam 值得質疑。