Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ncar!tank!staff_bob@gsbacd.uchicago.edu From: staff_bob@gsbacd.uchicago.edu Newsgroups: comp.ai Subject: Re: A Definition of "Symbol," "Symbolic," and "Symbol-Manipulation" Message-ID: <2308@tank.uchicago.edu> Date: 15 Mar 89 16:41:51 GMT Sender: news@tank.uchicago.edu Organization: University of Chicago Graduate School of Business Lines: 65 >From article , by harnad@elbereth.rutgers.edu (Stevan Harnad): ># ... ># the basis of EXPLICIT RULES that are (3) likewise physical tokens and ># ... i.e., it is purely SYNTACTIC, and consists of (5) rulefully COMBINING ># and recombining symbol-tokens. There are (6) primitive ATOMIC ... > >This characterization of 'symbol' is not appropriate for either >the ordinary use of the word or the way it is used in logic. >In the case of the latter, a logical semantics (e.g. the truth >table characterization of sentence logic) cannot have symbols, >by this account. > As usual, you've gotten me a little confused. I thought that Harnad gave a list of criteria for any logical formalism, in an effort to explain that by 'symbolic' we mean 'capable of being processed within the context of a formal system'. I thought that this was exactly the way the word 'symbol' is used in logic. Granted, transforming a simple system such as ordinary arithmetic into a purely symbolic system that meets these contraints is difficult, but it can be done. The intention here is that iff a system meets these constraints, then we know that it can be processed by a Turing Machine. Furthermore, after having stared at your point about "a logical semantics..." for more than a few minutes, I must confess that I simply don't understand it. If you're saying that the truth table characterization of sentence logic is not a symbolic system according to these rules, I might be prone to agree, but if you're saying that sentence logic "cannot have symbols", I disagree. For the most part, we don't work with purely symbolic systems, but, for logical purposes, it has generally been shown that it is in fact possible to characterize them by the rules Harnad has supplied. A truth table representation of a Boolean statement is easy for us to grasp conceptually, and it is certainly possible to represent a Boolean statement in a way to meet Harnad's criteria, so, where's the beef? (as an aside, in re-reading Simon's "Science of the Artificial", I noticed that he claims that by "symbol processing" he means the same thing that others mean by "information processing". It's not at all clear to me that the two are the same if we take the above mentioned definition of symbolic system.) >But that's ok -- we can take the proposed definition as giving >a special technical usage for purposes of discussion, though >we'll have to remember that if later the conclusion is drawn >that human thought is non-symbolic because it is based at least >in part on denotations, this follows trivially from the artificial >usage introduced earlier. Mustn't allow proof by definition. > >Does this definition help clarify whether brain-events evoked >by or associated with perception, or evoked in other ways, >can properly be called symbols? I don't see that it does. >Of course if speculation is not to be permitted, we could >point out that no one has given explicit rules which completely >characterize the way these brain-events interact, based >their physical form. Then the controversy is settled: human >thought is not symbolic. No explicit syntactic rules. > I don't think that's the point. The question is whether or not human thought can be characterized by a symbolic system, not whether or not it is one. The implication is that if it can, then we can model human thought on a digital computing device of some sort (i.e. a Turing Machine). R.Kohout