W3C home > Mailing lists > Public > www-rdf-logic@w3.org > April 2001

RE: newbie question

From: Danny Ayers <danny@panlanka.net>
Date: Mon, 23 Apr 2001 01:09:57 +0600
To: "pat hayes" <phayes@ai.uwf.edu>
Cc: <www-rdf-logic@w3.org>
This is going to take me a bugger of a long time to digest, but I have to
say now I appreciate it.

Danny Ayers

<- -----Original Message-----
<- From: pat hayes [mailto:phayes@ai.uwf.edu]
<- Sent: 23 April 2001 00:22
<- To: Danny Ayers
<- Cc: www-rdf-logic@w3.org
<- Subject: Re: newbie question
<- >Ok, I wanted to emphasise naivety.
<- >
<- >I am not a logician, but after muttering about the need for the
<- ability to
<- >have inheritance in metadata, I'm curious to know how something like
<- >inheritance fits in in formal logic terms - does it fall under
<- >qualification? (I am reading up on this, I assure you, though
<- it's taking me
<- >a while to absorb - I'd just like a predicate to be the same
<- thing from one
<- >doc to the next ;-).
<- OK. There's a couple of very basic ideas which re-appear in a number
<- of different forms. The ideas are basically a thing being in a set,
<- and one set being a subset of another.
<- First form: Talk about classes and instances. Object T is instance of
<- class C, and one class is subclass of another class.  Traditional
<- terminology in OO world.
<- Second form: Talk about things and sets. Thing T is member of set S,
<- and one set is subset of another set. Traditional terminology in
<- mathematics.
<- Third form: Talk about properties and individuals (= things).
<- Property P is true of thing T,(written (P T) in KIF syntax, P(T) in
<- more traditional logical syntax) and one property entails (or
<- implies) another property (ie if the first one is true of something
<- then so is the second.) Traditional terminology in formal logic. The
<- connection with the first two is through the fact that in formal
<- logic, a property is usually treated as pretty much the same as the
<- set of things it is true of, so you can say (Human Joe) or Joe is in
<- the set/class Human, and it means the same thing.
<- There are other forms as well, and these are sometimes combined
<- together, eg. some logicians talk about sorts and subsorts (= classes
<- and subclasses, but treated as similar to properties, and checkable
<- at parse time rather than having to do arbitrary inferences).
<- (One thing to note about the third form is that it doesnt have any
<- direct way to speak about the classes/properties, so it has to use
<- quantifers to say the equivalent of 'subclass'. Eg (A is subclass of
<- B) would look like this in logic, using KIF syntax:
<- (forall (?x)(implies (A ?x) (B ?x))), ie anything that is in A must
<- be in B.  Also, notice that this usage of the word "property" is
<- *different* from the usage in DAML+OIL, which uses the first form. In
<- the first form, "property" usually means something that relates
<- members of a class to other things, eg 'father' is a property of
<- human beings. In the other terminologies this would be called a
<- binary relation. )
<- Inheritance is the inference from something being true for a whole
<- class, to its being true for any subclass or instance. (Think of
<- classes and subclasses forming a tree with the universal class at the
<- top and the instances at the bottom, then inheritance moves
<- properties 'down' this tree.) So if you know that all mammals have
<- warm blood and you know that Aardvarks are mammals (or that Arthur is
<- a mammal) you know that Aardvarks (Arthur) has warm blood. This is
<- just normal logical inference in the third form, eg this example
<- would be
<- (forall (?x)(implies (Mammal ?x)(Warmblooded ?x)))
<- (forall (?x)(implies (Aardvark ?x)(Mammal ?x)))
<- infer
<- (forall (?x)(implies (Aardvark ?x)(Warmblooded ?x)))
<- In the first form it might look like this:
<- Mammal is subclass of Warmblooded
<- Aardvark is subclass of Mammal
<- infer
<- Aardvark is subclass of Warmblooded
<- Notice that saying 'subclass' and saying 'everything in a class has a
<- property' amount to the same thing, which is what you would expect if
<- you think about it. (Some combined forms would use one syntax for
<- subclass and a different one for saying warmblooded, but that is just
<- showing off.)
<- The most common way this is all said is that the first form
<- (class/instance) is used to talk about the classes that one is
<- proposing to use inheritance on in some special (efficient) way, and
<- anything else is said in the third form and called 'rules', which is
<- a non-logical way to say 'axioms'.
<- Inheritance is such a big deal for 3 reasons: (1) subclass
<- heirarchies tend not to change much, or at least not very often (2)
<- inheritance inferences are VERY common; and (3) inheritance
<- inferences can be managed very efficiently if they are separated out
<- from other inferences. (No, four reasons. (4) It is very handy to
<- state properties once somewhere in the heirarchy and not have to keep
<- repeating them over and over for every new thing in the database. So
<- inheritance inferences are necessary.) All of which suggests it is a
<- very good idea to put inheritance inferences into a special inference
<- black-box and compile it down to the metal, which is what everyone
<- wants to do.
<- DAML is influenced by this goal, but like most other languages it
<- also had to compromise. To get really, really fast inferences you
<- need to restrict the inheritance to a very simple case (just subclass
<- and instance, basically) and people tend to want to say more
<- complicated things (eg property restrictions in DAML) and as soon as
<- you start doing that, it gets harder to keep all the relevant
<- inferences inside the box and keep the box running very fast.
<- (Actually, strictly, it gets hard to *guarantee* that the box will
<- run fast. In practice, one can get quite useful speeds out of quite
<- an expressive box almost all the time, though you do have to be
<- careful.)
<- Hope this helps some.
<- Pat Hayes
<- PS. On "predicate": logicians tend to make a very strict distinction
<- between things and names of things.  You will often find that one
<- will uses "predicate" only to refer to *names*, ie its a kind of
<- symbol, but others will use "predicate" to mean what the symbol
<- refers to, which would be what I'm calling a property. If you read
<- one guy's prose with the other one's meanings you will get very
<- confused, so find out exactly what they mean and bear it in mind when
<- reading.
<- ---------------------------------------------------------------------
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<- phayes@ai.uwf.edu
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Received on Sunday, 22 April 2001 15:14:04 UTC

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