Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!seismo!lll-lcc!lll-tis!ames!sdcsvax!ucbvax!SAIL.STANFORD.EDU!JMC From: JMC@SAIL.STANFORD.EDU (John McCarthy) Newsgroups: comp.ai.digest Subject: Natural kinds Message-ID: <8707150640.AA19353@ucbvax.Berkeley.EDU> Date: Fri, 10-Jul-87 13:19:00 EDT Article-I.D.: ucbvax.8707150640.AA19353 Posted: Fri Jul 10 13:19:00 1987 Date-Received: Fri, 17-Jul-87 03:30:36 EDT Sender: daemon@ucbvax.BERKELEY.EDU Distribution: world Organization: The ARPA Internet Lines: 61 Approved: ailist@stripe.sri.com Recently philosophers, Hilary Putnam I think, introduced the concept of natural kind which, in my opinion, is one of the few things they have done that is useful for AI. Most nouns designate natural kinds, uncontroversially "bird", and in my opinion, even "chair". (I don't consider "natural kind" to be a linguistic term, because there may be undiscovered natural kinds and never articulated natural kinds). The clearest examples of natural kind are biological species - say penguin. We don't have a definition of penguin; rather we have learned to recognize penguins. Penguins have many properties I don't know about; some unknown even to penguin specialists. However, I can tell penguins from seagulls without a precise definition, because there aren't any intermediates existing in nature. Therefore, the criteria used by people or by the programs we build can be quite rough, and we don't all need to use the same criteria, because we will come out with the same answer in the cases that actually arise. In my view the same is true of chairs. With apologies to Don Norman, I note that my 20 month old son Timothy recognizes chairs and tables. So far as I know, he is always right about the whether the objects in our house are chairs. He also recognizes toy chairs, but just calls them "chair" and similarly treats pictures of chairs in books. He doesn't yet say "real chair", "toy chair" and "picture of a chair", but he doesn't try to sit on pictures of chairs. He is entirely prepared to be corrected about what an object is. For example, he called a tomato "apple" and accepted correction. We should try to make AI systems as good as children in this respect. When a an object is named, the system should generate a gensym, e.g. G00137. To this symbol should be attached the name and what the system is to remember about the instance. (Whether it remembers a prototype or a criterion is independent of this discussion; my prejudice is that it should do both if it can. The utility of prototypes depends on how good we have made it in handling similarities.) The system should presume (defeasibly) that there is more to the concept than it has learned and that some of what it has learned may be wrong. It should also presume (although will usually be built into the design rather than be linguistically represented) that the new concept is a useful way to distinguish features of the world, although some new concepts will turn out to be mere social conventions. Attaching if-and-only-if definitions to concepts will sometimes be possible, and mathematical concepts often are introduced by definitions. However, this is a rare case in common sense experience. I'm not sure that philosophers will agree with treating chairs as natural kinds, because it is easy to invent intermediates between chairs and other furniture. However, I think it is psychologically correct and advantageous for AI, because we and our robots exist in a world in which doubtful cases are rare. The mini-controversy about penguins can be treated from this point of view. That penguins are birds and whales are mammals has been discovered by science. Many of the properties that penguins have in common with other birds have not even been discovered yet, but we are confident that they exist. It is not a matter of definition. He who gets fanatical about arbitrary definitions will make many mistakes - for example, classifying penguins with seals will lead to not finding tasty penguin eggs.