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.