248 13 Local, Global and Glocal Knowledge Representation
indicates the simple and uninteresting ConceptNode grouping three objects owned by Ben (note that the above-given Atoms don’t indicate the ownership relationship, they just link the three objects with textual descriptions). In this example, the ConceptNode links transparently to physical objects and English descriptions, but in general this won’t be the case — most ConceptNodes will look to the human eye like groupings of links of various types, that link to other nodes consisting of groupings of links of various types, etc.
There are Atoms referring to basic, useful mathematical objects, e.g. NumberNodes like
NumberNode #4 NumberNode #3.44
The numerical value of a NumberNode is explicitly referenced within the Atom.
A core distinction is made between ordered links and unordered links; these are handled differently in the Atomspace software. A basic unordered link is the SetLink, which groups its arguments into a set. For instance, the ConceptNode C defined by ConceptNode C
MemberLink A C MemberLink BC
is equivalent to
SetLink AB
On the other hand, ListLinks are like SetLinks but ordered, and they play a fundamental role due to their relationship to predicates. Most predicates are assumed to take ordered arguments, sO we may say €.g.
EvaluationLink PredicateNode eat ListLink
ConceptNode cat ConceptNode mouse
to indicate that cats eat mice. Note that by an expression like
ConceptNode cat
is meant
ConceptNode C ReferenceLink WC WordNode W #cat
since it’s WordNodes rather than ConceptNodes that refer to words. (And note that the strength of the ReferenceLink would not be 1 in this case, because the word "cat" has multiple senses.) However, there is no harm nor formal incorrectness in the "ConceptNode cat" usage, since "cat" is just as valid a name for a ConceptNode as, say, "C."
We've already introduced above the MemberLink, which is a link joining a member to the set that contains it. Notable is that the truth value of a MemberLink is fuzzy rather than probabilistic, and that PLN is able to inter-operate fuzzy and probabilistic values.
SubsetLinks also exist, with the obvious meaning, e.g.
ConceptNode cat
ConceptNode animal SubsetLink cat animal
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