From: pat hayes (phayes@ai.uwf.edu)
Date: 02/07/01
>[Dan, to Peter:] >I'm not sure what you mean by "the definition of a property". >Do you mean that there's always exactly one such definition? >how much stuff goes in a definition? I know what documents >are and I know what statements/formulas/assertions are. >Maybe I can read "a document containing statements relevant >to some property" in place of "the definition of a property." Let me suggest that a better phrasing might be "a collection of statements which together are sufficient to define a property". The point being that while these might not be all in one place (the web being what it is) and while they may need to assembled in a piecemeal fashion, nevertheless they do all need to be somehow got together before certain operations can be done reliably (that is, one can jump ahead without having them all, maybe, but at the risk of needing to later make corrections, whereas if one knows that one has them all in hand, one can be sure that the next step is sound.). Analogy might be the packets which comprise an email message, which have to be all gathered together to reconstruct the message reliably in toto, though one could make do with only some of them in an emergency (but would later have to move things along to fill in the gaps, etc.) Your beef with Peter seems to be, if I follow you, that since we have to allow 'extendability' by adding further information (since it might be scattered around anywhere and we might find it at any time, this being a pull technology, etc.) that the web-logic cannot permit the use of such 'required' conditions. But I think there are two issues: one, we may have to gather data where we find it and have to take it as it comes, and two, we shouldnt always be in a nonmonotonic position of maybe having to undo things that have been done. One can have both of these, at the cost of delaying some things until one is certain one knows what to do. But delaying isn't the same as undoing. >As to why not defer parsing... a major goal of all this >logic-and-the-web stuff, to me, is a certain sort of scalability >where we can analyze documents independently, see >what they mean (i.e. what formulas they state) and then >merge information from multiple documents in a monotonic >fashion. This is a major feature of RDF. It's something I >would require even if we threw out RDF syntax and started over >with XML and URIs. But if we take this position very strictly, and if a 'document' can be a sufficiently small piece of information, then it becomes impossible to say anything nontrivial. You will HAVE to compromise on this (a little) or web logic is never going to get off the ground (or else you will have to allow a 'document' to have a certain minimal complexity, so that the content comes in sufficiently well-organized chunks.) Some content can be rendered down into a huge conjunction of small parts, but a lot of it - almost anything that might be called 'structured data' - cannot be. You can't for example express the content of a quantified statement that way, or a reasonably large negation, let alone things like images or arrays or most programming language expressions. >To have the parsing of one document depend on the >contents of another conflicts with that goal/principle. Suppose that one document contains a large collection of declarations of a vocabulary of symbols, and someone wants to write another document which uses that vocabulary, in some reasonably expressive language. Your requirement has the consequence that it is impossible to parse the second document, in general, since without the declarations the parser doesnt know how to proceed. One way around the letter, but not the substance, of this objection is to make the 'language' of these documents be not the content language, but a meta-language of that language. This is what you are doing with RDF (for anything other than a conjunction of ground atoms.) Since, on the whole, *expressions* of languages are fairly simple structures (directed labelled graphs will probably do the trick, with a little tinkering), one canhave a uniform syntax for this meta-language which will enable one to describe any expressions in any language (like XML); and since this is universal, one can write a parser for it, and bob's your uncle: you can parse the second document without looking at the first document (in fact, without looking at ANY other documents.) Well, yes; but also no, since you are, remember, now working in a metalanguage, not the content language. Parsing an expression in the metalanguage only gives you a *description* of the expression you wanted originally: it doesn't give you the expression itself. You still need to parse *that* in a way that uncovers enough of its structure (eg variable bindings, scopes of complex operators, etc.) to enable your reasoners/interpreters/whatever do their stuff properly, and you still can't do that without reference to the first document, because the relevant information just can't be gotten any other way (except maybe by prayer). >Another way to state this principle is that >the knowledge contained in two documents, X and Y, >is always the conjunction of the knowledge in X with >the knowledge in Y. To allow X to change what Y says >in some non-monotonic way doesn't seem scalable/workable >to me. two points: (1) you need to get clear about what counts as a document and what you take X and Y to be *about*. (content language or meta-language, see above.) If 'documents' are any amount of information about anything, then this is an impossible requirement. (Suppose I want to say 'either X or Y'. If I conjoin X or Y then I've said too much; if I don't include them then nobody is going to know what I'm talking about.) (2) Jim Hendler is right. If the meaning of Y depends in part on the meaning of X, that doesnt automatically produce nonmonotonicity. It just means that one has to use X to figure out the meaning of Y. The monotonicity would be produced, maybe, if one insisted on making assumptions about the meaning of Y without knowing X, which might later turn out to be wrong. But sometimes you can wait until you do know, or report back to something else that you can't make progress until you are told, or whatever. I agree these complexities make for boring reading, but I don't think that there is any way out, in general: we are going to have to deal with things like this whether we want to or not, ultimately because INFORMATION is like this. It just is, however we encode it. Life is not a monotonic conjunction of cherries. > > In particular, it seems to me that your proposal has exactly the same > > problem. You also depend on external information on how properties should > > work. > >But the various bits of information accumulate in >the normal monotonic fashion; I don't have >the situation where I initially parsed it as >a string, but then I discover I was wrong or something >and I have to undo stuff. If you don't parse until you know how to parse, you won't get into this position. So what you really want, I think, is to know (locally and absolutely, ie monotonically with respect to new info. from elsewhere) whether or not you do have enough information to parse monotonically. (In many languages, for example, a parser on finding an identifier will look it up in a table of declared forms and if it isnt there, will post an error condition and refuse to compile.) But that's not the same as requiring that you must have, locally, enough information to parse. Same point applies to other things as well as parsing, of course. Pat Hayes PS. Dan, thanks for stating these design principles in black and white. I think that more discussions at this level about what people see as their over-riding aims and basic assumptions would be very useful. Knowing them would have been very helpful to me in understanding the rationale for the RDF design decisions, for example. PPS. Any objections to putting this discussion onto rdf-logic? It seems to belong there. --------------------------------------------------------------------- IHMC (850)434 8903 home 40 South Alcaniz St. (850)202 4416 office Pensacola, FL 32501 (850)202 4440 fax phayes@ai.uwf.edu http://www.coginst.uwf.edu/~phayes
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