Web Ontology Language (OWL)

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21 Οκτ 2013 (πριν από 4 χρόνια και 23 μέρες)

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© 2005-2006 The ATHENA Consortium.
Web Ontology Language (OWL)
• A more expressive ontology language
• Concepts (classes) can be described or defined
– described – necessary conditions given
– defined – necessary and sufficient conditions
given
• Builds on RDF and can be expressed in several ways:
– RDF XML-based syntax
– abstract syntax
– graphic UML-like
• Has three sub-languages:
– OWL Full
– OWL Description Logic (DL) – maps to a DL, a
subset of predicate logic
– OWL lite – for simple taxonomies (class
hierarchies)
Let’s continue with The Web Ontology Language, OWL for short. an OWL is a bird, the symbol of
Athena, the goddess of wisdom. OWL is an ontology language which is more expressive than RDF.
Like RDF it can be used to define the terms to be used in RDF models.
Using OWL classes (concepts) can be either described or defined.Described means giving only
necessary conditions; defined means giving necessary and sufficient conditions. Let’s recall what
we learned about necessary and sufficient.
Every class member fits the necessary description, but there may be things that fit the description
that are not members of the class. Recall we can describe
man as a featherless biped, but a
plucked chicken also fits that description.
Necessary and sufficient conditions mean that if something matches the conditions it is
a member
of the class. It is difficult to determine necessary and sufficient conditions.
OWL builds on RDF; it uses terms from RDF like subclass. OWL can be expressed in several
ways: using RDF XML-syntax, using an abstract syntax, or using a graphical syntax similar to UML,
for those who know that.
OWL has 3 sub-languages, OWL Full, OWL description logic, and OWL lite used to create simple
taxonomies of class hierarchies.
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© 2005-2006 The ATHENA Consortium.
Reasoning
Socrates:
“In that case, if Eros is lacking in what is
beautiful and what is good is beautiful, he will
also be lacking in what is good.”
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1. Plato, Symposium, edited by C.J. Rowe, Aris & Phillips Classical Texts, 1998, p.77
premises:
Eros is lacking in what is beautiful
What is good is beautiful
conclusion:
Eros is lacking in what is good
v:Eros
v:Beautiful
v:has
Most people use OWL DL because there are reasoning tools that can handle OWL DL, and which
have good performance.
I want to spend the rest of this section on an example of reasoning using OWL. The example
comes from Plato’s symposium, in which Socrates says: “In that case, if Eros is lacking in what is
beautiful and what is good is beautiful, he will also be lacking in what is good.” This can be
analyzed into the premises: Eros is lacking in what is beautiful, and what is good is beautiful. The
first premiss can be drawn using an instance diagram in a slightly unusual way to show that it is
not the case that Eros has anything that is Beautiful. To simplify matters, I have shown Eros in a
class of his own.
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To emphasize that Eros has nothing beautiful, I have move the arrow to the edge of the Beautiful
class. Taking into account the second premiss, what is good is beautiful, I would like you to
complete this picture.
What will this instance diagram look like if you add the class of good things?
© 2005-2006 The ATHENA Consortium.
Question 3.3
What will this instance diagram look like if you add the class
of Good things?
Hint: recall Euler diagrams.
premises:
Eros is lacking in what is beautiful
What is good is beautiful
conclusion:
Eros is lacking in what is good
v:Eros
v:Beautiful
v:has
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© 2005-2006 The ATHENA Consortium.
Answer 3.3
The key to the answer is to recognize that to say that ‘what
is good is beautiful’ is the same as saying that ‘Good is a
subclass of Beautiful’.
premises:
Eros is lacking in what is beautiful
What is good is beautiful
conclusion:
Eros is lacking in what is good
v:Eros
v:Beautiful
v:has
v:Good
You should have drawn good inside of beautiful to show that it is a subclass of it. The key to the
answer is to recognize that saying that what is good is beautiful is the same as saying that good is
a subclass of beautiful. You can now see that the conclusion is true, that is, it is not the case that
Eros has something that is good.
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© 2005-2006 The ATHENA Consortium.
We can get a reasoning engine to reach this same conclusion that we saw by giving it the
premises.
This is a screen shot form the Protege OWL editor showing the 2
nd
premiss of the argument, good
is a subclass of beautiful. This is shown in the subclass relationship pane by the fact that good is
indented relative to beautiful.
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© 2005-2006 The ATHENA Consortium.
At the very bottom of this window you see the 1
st
premiss, the fact that it is not the case that Eros
has something from the Beautiful Class. That funny hook means NOT and the backward E means
exists, the has
is a property, the same has
property we used in the instance diagram.
So, if we read that last line literally, we get NOT, meaning it is not the case that, there exists a has
relation between Eros and the Beautiful class. We brought in Eros because Eros is selected in the
class pane, so everything we see here is some fact about Eros, for example Eros is a subclass of
owl thing. The sideways u with the line underneath in yellow is the symbol of subclass. So we read
the line above the last as Eros is a subclass of owl thing. You can think of the process of editing an
OWL ontology as a process of creating a set of facts about classes.
A reasoner can use these facts to draw as conclusion more facts.
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© 2005-2006 The ATHENA Consortium.
Question 3.4
premises:
Eros is lacking in what is beautiful
What is good is beautiful
conclusion:
Eros is lacking in what is good
Why did I say that Eros
was described rather
than defined?
Why did I say that Eros was described rather than defined?
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© 2005-2006 The ATHENA Consortium.
Answer 3.4
premises:
Eros is lacking in what is beautiful
What is good is beautiful
conclusion:
Eros is lacking in what is good
Why did I say that Eros
was described rather
than defined?
because the conditions
are only necessary.
Hopefully, you noticed that the conditions were under the necessary category and not the
necessary and sufficient. Remember that necessary conditions give us a superclass of the class
we are describing. That’s the reason for the subclass symbol, which can be read as saying that
Eros is a subclass of the class of things that fit the conditions.
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© 2005-2006 The ATHENA Consortium.
As a next step we need to define what it means to be lacking in good, since we are trying to
conclude that Eros belongs in that class. This time we really define rather than describe the class.
We will call it NonGood and define it as all those things that do not have anything good.
Notice that these conditions are in the necessary and sufficient section of the asserted conditions
pane.
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© 2005-2006 The ATHENA Consortium.
Protege: Eros classified by reasoner
premises:
Eros is lacking in what is beautiful
What is good is beautiful
conclusion:
Eros is lacking in what is good
Now we let the reasoner work and inspect the results. We see in the second subclass relationship
pane that Eros, in blue, has been classified by inference to be a subclass of NonGood, which we
defined to be those things lacking in Good.