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Paper: strengthen Section 4.1
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paper.tex
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paper.tex
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@ -365,9 +365,15 @@ Consider: ``The gear on the left turns clockwise. It meshes with the
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middle gear, which meshes with the gear on the right. Which direction
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middle gear, which meshes with the gear on the right. Which direction
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does the right gear turn?''
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does the right gear turn?''
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To answer, you must simulate the mechanism. Left gear clockwise $\to$
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To answer questions like this with 100\% accuracy,
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you must simulate the mechanism. Left gear clockwise $\to$
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middle gear counterclockwise (meshing reverses direction) $\to$ right
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middle gear counterclockwise (meshing reverses direction) $\to$ right
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gear clockwise. You cannot determine this by inspecting the words. You
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gear clockwise. You cannot determine this by inspecting the words and
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expect the result to be accurate in all cases. Suppose for example you
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tried to use a simple heuristic like ``every mention of `gear' flips the
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answer'': that would fail as soon as somebody replaced ``which'' with
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``and that gear''. Other heuristics may survive more variations, but to
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get it right in 100\% of cases you need to model the semantics. You
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must run the described process in your head, stepping through the causal
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must run the described process in your head, stepping through the causal
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chain. Add more gears, add branching gear trains, and the computation
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chain. Add more gears, add branching gear trains, and the computation
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becomes arbitrarily long --- but the structure is the same. The sentence
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becomes arbitrarily long --- but the structure is the same. The sentence
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@ -383,10 +389,17 @@ program. Understanding it means running it.
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Rice's Theorem (1953) makes this precise: no non-trivial
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Rice's Theorem (1953) makes this precise: no non-trivial
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semantic property of Turing-complete programs is decidable without
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semantic property of Turing-complete programs is decidable without
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running them. You cannot determine what a program does by inspecting it.
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running them. You cannot determine what a program does by inspecting it
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and be 100\% correct in finite time no matter what the input. You can
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have heuristics that work {\em some} of the time, and even formal proof
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methods that work for {\em some} inputs, but no inspection can survive
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100\% of programs if a 100\% accuracy is required.
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You must execute it. Natural language has Turing-complete expressive
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You must execute it. Natural language has Turing-complete expressive
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power. Therefore you cannot determine what a natural language utterance
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power. Therefore you cannot determine what a natural language utterance
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\textit{means} without executing the computation it describes.
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\textit{means} without executing at least some of the computation it
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describes. (You can understand the Ackermann function without having to
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compute the whole thing, but you'll need at least a demonstrative run of
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a few steps to understand its pattern.)
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The halting problem tells us the same thing from a different angle.
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The halting problem tells us the same thing from a different angle.
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A system that could determine the meaning of arbitrary natural language
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A system that could determine the meaning of arbitrary natural language
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