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.
以上是节录一段网站桶中脑的文章
我觉得premise1 是个有问题的句子
premise1: If I know that P, then I know that I am not a brain in a vat
请问大家 "如果我知道P 那么我不是桶中脑" 这句为什么会成立?
我看了版上关于桶中脑的讨论 都没有人质疑Putnam 我觉得很奇怪
※ 引述《aletheia (HERESY)》之铭言：
: 桶中大脑（BIV）。
: 常被用来当作反对外在世界存在的例子。
: 就形上学来说，BIV的确反对形上学的实在论(MR)
: 但以知识论来说，BIV不反对知识论上的实在论(ER)
: 何解？

前提 1 是怀疑论命题的逻辑等值语句，怀疑论命题是：If I do not know that I am not a B.I.V., then Idon't know that P (given any P).将怀疑论命题以异质位换（contraposition）将前后件位置对调并各自加上否定后，便得前提 1。如果你认为前提 1 有问题，那么其实就是认为怀疑论命题有问题，也就是认为怀疑论的结论是有问题的。另外，前提 1 并不是 Putnam 的主张，所以我也不清楚为什么你会认为前提 1 值得怀疑就意味着 Putnam 值得质疑。