CC-BY
Fabian M. Suchanek
Knowledge
Representation
70
We want to be able to represent any possible bit of knowledge
about the world in a computer. No easy feat:
Goal: Make computers understand
2
—
industry experts from Google, Microsoft, Facebook,
Amazon, IBM
Knowledge representation is a difficult skill to learn on
the job. [...] The importance of knowledge representation
in diverse industry settings [...] should reinforce the
idea that knowledge representation should be a
fundamental part of a computer science curriculum as
fundamental as data structures and algorithms.
Here, we look at the knowledge representation formalism
that has evolved as a standard in the domain.
Overview
3
•
Entities
•
Classes
•
Relations
•
The gory details
•
Reification
•
Canonicalization
•
The Open World Assumption
•
Reality
An
entity
(also: resource) is any particular object of the world
or imagination, be it abstract or concrete.
Entity
4
(The exact definition of “entity” is a philosophical conundrum. Don’t worry about it for this course.)
>digression
Digression: Entities
5
Is this an entity?
How many entities are there?
Or this?
Digression: Entities
6
Is this an entity?
How many entities are there?
Or this?
Isn’t everything just atoms?
>digression
Over time, all parts of a ship are
replaced at some point of time.
Then, is it still the same ship?
see: Theseus’s ship on Wikipedia
Digression: Identity
7
New York Times
Over time, all parts of a ship are
replaced at some point of time.
Then, is it still the same ship?
Digression: Identity
8
Humans replace their cells every 7 years.
see: Theseus’s ship on Wikipedia
A KB usually contains only a finite set of entities that are of interest, and it
assumes them to be atomic and identifiable.
An
identifier
for an entity is a string of characters
that represents the entity uniquely in a KB.
Def: Identifiers
9
Examples for identifiers:
• unique names, as in YAGO or DBpedia:
Elvis_Presley_(singer)
• abstract identifiers, as in Wikidata or Freebase:
/m/02jq1
We sometimes say “entity” when we mean the identifier. We sometimes use images for the identifiers.
Try it out!
An
label
for an entity is a human-readable string that names the entity.
Labels that refer to the same entity are called
synonyms
.
Entities that have a label are called
named entities
.
Def: Labels
10
identifier (for a named
entity, Elvis Presley)
labels
(all synonymous)
[Elvis on Wikidata]
A label that refers to several entities is called
ambiguous
.
Def: Ambiguity
11
Example: “Paris” is an ambiguous label, as it can refer to
several cities, a greek hero, or people with that name.
Paris reads about Paris in Paris.
Try it out!
Identifier:
Paris_Hilton
Paris_(Greek_myth)
Paris_(city)
Labels:
“Paris Hilton”
“Paris Whitney Hilton”
“
Paris
”
...
“Paris the Hero”
“
Paris
”
“Alexander”
...
“
Paris
”
“City of Light”
“Parigi”
...
>literals
A
literal
is a fixed value that takes the form of a string of characters.
(It is an entity that is identical to its identifier — in all KBs.)
Def: Literals
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"1935-01-08"
"Hello world"
"42"
This is the number 42,
and any KB will use the identifier "42"
to refer to this entity.
Overview
13
•
Entities
•
Classes
•
Relations
•
The gory details
•
Reification
•
Canonicalization
•
The Open World Assumption
•
Reality
A
class
(also:
concept
) is a set of similar entities. Each entity is an
instance
of (also: has the type of, belongs to) the class.
Classes:
- Actors
- Cars
- Cities
- Rivers
- Universities
- Theories
- ...
Class
14
(The exact definition of “class” is a philosophical conundrum, too. See later in this lecture.)
Instances
Class “Actors”
Classes: Example in YAGO
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