Peter Szolovits
Lowell B. Hawkinson
William A. Martin
MIT/LCS/TM-86
June, 1977
Massachusetts Institute of Technology
Laboratory for Computer Science
(formerly Project MAC)
Cambridge Massachusetts, 02139
We describe the motivation and overall organization of the OWL language for knowledge representation. OWL consists of a memory of concepts in terms of which all English phrases and all knowledge of an application domain are represented, a theory of English grammar which tells how to map English phrases into concepts, a parser to perform that mapping for individual sentences, and an interpreter to carry out procedures which are written in the same representational, formalism. The system has been applied to the study of interactive dialogs, explanations of its own reasoning, and question answering.
Keywords: Artificial Intelligence, Knowledge Representation, LMS, Memory Structures, Natural Language, OWL, Symbol Manipulation
This work was supported by the Advanced Research Projects Agency of the Department of Defense and was monitored by the Office of Naval Research under contract #N00014-75-C-0661.
We wish to thank our colleagues and students in the Laboratory for Computer Science and the Artificial Intelligence Laboratory at MIT for their contributions to our understanding.
This paper was presented at the Workshop on Natural Language for Interaction with Data Bases held by the International Institute for Applied Systems Analysis (IIASA) at Schloss Laxenburg, Austria, in January, 1977. It will also appear in the proceedings of that Workshop [Rahmstorf 1977].
I. Overview and MotivationII. The Linguistic Memory System
A. The Essential Structure of ConceptsIII. How English Phrases are Represented as Concepts
B. Attachment
A. What Does an English Expression Say?IV. Reasoning
B. Kinds of Specialization
C. Parsing
Bibliography
We have undertaken the design and implementation of a new computer language for knowledge representation, called OWL. We have become convinced that recent progress in Linguistics and in Artificial Intelligence (AI) now suggests a set of principles which are worth implementing as part of a programming language to make them uniformly accessible for our further work.
For a computer program--as for a person--it is more effective to know how to do something than to be able to figure it out. The AI field has made important progress under an opposite set of assumptions: that all knowledge of the domain should be expressed in propositional form and that a program's actions should be directed by a general-purpose problem solver operating on propositions representing the application world. Such a problem solver would always figure out what to do next based on the state of the world and its set of general principles. At the same time, most programs which have been used for their ability to perform in an application domain rather than for their pedagogic clarity have used a very different form of organization: the knowledge of how to perform the task was implicitly built into the steps of the program. Of course, such an organization is generally accompanied by inflexibility, difficulty of extension, incomprehensibility and unprovability of the program, and many other ills. If, however, we could express the description of the procedural knowledge of the program in the same formalism as its declarative knowledge of the domain of application, then both would be equally accessible.
This is precisely what is done in OWL--the program is just another aspect of the description of the application world, and knowledge of how to solve specific problems of that world can be explicitly embedded in the description.
We have taken English as the basis for our knowledge representation formalism. The greatest attraction of this approach is that it almost trivially satisfies our need for expressive power. After all, native speakers of English can usually communicate their knowledge of any domain of interest in English,1 perhaps augmented by specialized notations and vocabularies particular to the domain. Because we choose a computer representation which is designed to be similar to the natural language employed by a computer-naive user of one of our programs, we expect that the translation process from English sentences to our internal structures will be straightforward. Once we succeed in translating the English phrase into our internal representation, that will allow all of OWL's activities, including understanding the sentence in semantic detail, resolving references, mapping the sentence onto some capability of the system for acquiring new knowledge or answering on the basis of old, etc., to make use of the same representational formalism. This, in turn, will help us to make the complete operation of the program accessible for explanation to, and modification by, someone who may well understand the domain of application but not our computer technology.
Arguments for English as a programming language have been made since the early 1960's, yet it has not been universally acclaimed as desirable. The principal objections to basing a programming language on English (or any natural language) center on the innate ambiguity of natural language and its lack of conciseness when contrasted with special mathematical notations. The second problem is rapidly resolved if we extend our definition of natural language to allow the incorporation of new notations. After all, the natural language of a physics text is hardly the literary English of the day. The first problem has both a trivial and a difficult component: pure syntactic ambiguity, as created by the existence of homonyms for instance, is simply controllable, whereas ambiguity arising from the fact that what one (literally) says is not what one actually means is, of course, difficult. Our response is simply that we wish to begin by representing precisely what one says, and we will allow the determination of the meaning of each utterance to be part of the problem that the system is to solve.
During the past few years, we have implemented the following components of a complete system based generally on the above ideas:
In terms of the above components, we have been building the following programs:
We will give an overview of LMS, the theory of grammar, and the interpreter, and discuss other modules as they relate to those central components.
The OWL LMS is a semantic network with a single primary data type, the concept, and a secondary data type, the symbol. Symbols are merely strings of characters which denote senses of English words and affixes and have no innate significance. Concepts represent the meanings of all words, phrases, clauses, sentences, etc. of English as well as any needed non-linguistic entities. It Is very important to note that, whereas in a traditional semantic network each node of the network represents a single word or item, in LMS each node represents any of the higher-level constructions mentioned above. Thus, where a typical semantic net would Identify the meaning of a sentence as some subnet of the whole network, LMS identifies it as a single node of the network.
Concepts, the nodes of LMS, have structure. In fact, we will concentrate on the essential structure of a concept as the primary organizational facility of LMS.
Every concept is defined by a pair, (genus
specializer), the essence of that concept. The
genus is another concept, and the specializer is either a concept or a
symbol. The genus specifies the general type of the concept; if the
genus of concept C is B (i.e., if C =
(B specializer)), then we imply that C
is-a B, or C is a kind of
B.2 C is called a specialization of
B, and B is called a generalization
of C. The specializer serves to distinguish this concept
from all other concepts with the same genus; it does not by itself
define the concept.3 The genus and the specializer together identify a
concept.
We want to interpret all the concepts in LMS as forming a single
taxonomy or tree-like classification system in which the genus points
"up" in the taxonomy. To do so, we must designate a single concept,
SUMMUM-GENUS, whose genus is itself. That condition makes
SUMMUM-GENUS the root of the tree. Further, we insist
that no loops may occur in the expression of concepts in terms of
themselves or each other (with the above exception for
SUMMUM-GENUS). Then, all concepts will form a tree
structured classification: starting from any concept in the conceptual
memory and successively moving to its genus will always lead to the
root concept SUMMUM-GENUS in a finite number of
steps. That number will be called the genus depth of the
concept. We also introduce a notational convenience. So far, we have
only allowed a concept to be written as (genus
specializer). But clearly, the depth of parenthetization for
writing any concept will be at least its genus depth, and this is
terribly inconvenient. Thus, we allow equivalence declarations, such
as A = (B C), which allows any appearance of
A to stand for an appearance of (B
C). A is called the label of (B
C).
The notion of derivative subclassification [Hawkinson 1975]
complicates this picture somewhat. It assures that all specializations
of a concept are classified the same way the specializers themselves
are classified in the conceptual memory. For example, if in the
taxonomy both DOG and PIG have genus
ANIMAL, then we classify (TAIL DOG) and
(TAIL PIG) under (TAIL ANIMAL). The
generalizer of a concept (A B) is the most specific
specialization of A whose specializer is a generalization
of B, or, if there are none of these, just A
itself.4 The genus of a concept is thus always either its generalizer
or the generalizer of its generalizer, etc. By moving along the
successive generalizers from any concept, we must finally reach
SUMMUM-GENUS, and the number of steps required is called
the generalizer depth of the concept.
We have now described some of the essential structure of each concept, thus each node, of a conceptual memory. Before we turn to arguing for the utility of this structure to represent knowledge, let us see what the essential structure of the nodes already implies for the semantic network as a whole. In our current implementation, every concept is directly linked to its generalizer and specializer. Every concept is not, however, linked directly to its genus, since the genus can easily be computed from generalizer and specializer links. A typical, but very small, conceptual memory taxonomy is shown in Figure 1.
SUMMUM-GENUS
NOUN
MASS-NOUN
WATER
SCALE
HEIGHT
(HEIGHT JOHN)
WEIGHT
COUNT-NOUN
TREE
(TREE APPLE)
ANIMAL
DOG
FIDO
PIG
PERSON
JOHN
PROFESSOR
APPLE
TAIL
(TAIL ANIMAL)
(TAIL PIG)
(TAIL DOG)
(TAIL FIDO)
ADJECTIVE
EMPTY
TRUE
VERB
MODAL
WILL
AUXILIARY
BE
NON-MODAL-AUXILIARY
TRANS
GO
HIT
ACT
ATTACH
Figure 1. A Sample Conceptual Memory Taxonomy This figure shows the classification of some of the concepts used
in this paper into a small conceptual memory taxonomy. The taxonomy is
a tree which is shown in the figure by successively indenting
branches, as in an outline. Note that derivative subclassification
causes the subtree under |
In the previous section, we presented the essential structure of a
concept in LMS. The act of creating a new node in LMS is called
specialization, and we say that we specialize a genus, G,
by a specializer, S, to form the concept (G
S). As we shall argue, any phrase of English can be suitably
encoded as a single concept (though of course it may be a very complex
one). When we wish to reason with a concept, however, we will find it
convenient to introduce an epistemologically distinct meta-level
representation. For example, if the concept C encodes the
sentence "John Smith is a good man" and we wish to represent our
belief that C is true, we cannot merely encode with
D that "That John Smith is a good man is true," because
now the question of D's truth is open for discussion.5 We
retreat to a formal meta-level to make statements about elements of
our universe of discourse, which are to be taken at face value rather
than be subject to interpretation. With such an ability for meta-level
description, we see that if C is marked as
TRUE at the meta-level, then that is a stronger statement
than D. From the former, the Interpreter may conclude
C's truth absolutely, while from the latter, only
conditionally on D's truth.
The act of attachment creates a directed link in LMS between
two nodes. We write [A B] and say that B is
attached to A. Unlike specialization, attachment
creates no new concepts. It merely establishes an (unlabelled) link
from A to B. The meaning of that connection
will depend completely on what A and B are
and on whatever is interpreting the connection. We give a few
illustrative examples of attachment here:
B whose generalizers are the concept
A are automatically attached to A and are
called its indexed branches because they are classified directly under
A in the specialization taxonomy.
(C A) are attached to the concept
A and are called its indexed aspects. For example,
(AGE JOHN) may be attached to JOHN and
encodes JOHN's AGE aspect.
Note that both of the above forms of attachment are easily recognizable because the concept to which attachment is made appears as the generalizer or specializer, respectively, of the attached concept. They derive from the essential structure of concepts and serve much the same purpose for the conceptual memory as do index entries in a book's index. These attachments do not really bear information; they are established when the taxonomy is built and are not subject to interpretation or change. Thus, the use of attachment, a meta-level operation, is appropriate.
[(AGE JOHN) 49].
[JOHN MIDDLE-AGED], [(AGE JOHN) (EQUAL (TO (AGE MARY)))].
[JOHN PROFESSOR].
This second set exemplifies storage of information (facts in the object domain), yet we are representing such information at the meta level. This is because we intend that reasoning be based on these facts without further verification. We are willing to guarantee their truth in this domain of application.
In this section, we shall first argue informally that the combination of concepts through specialization provides a mechanism capable of representing English phrases. We will then extend our notion of specialization to deal more rigorously with some problems we encounter.
We view English phrases as expressions built up by combination. To explore what forms of combination are necessary, we examine some modes of communication in English and see how they are achieved by combining words and phrases.
APPLE = (FRUIT "APPLE").6 But we need
many more conventional names than we have words in our language. So,
we permit the formation of conventional names as combinations
(pairs). One member of the pair indicates the class of the concept,
the other provides a distinguishing, or specializing, element to make
the pair unique. For example, "apple tree" Is a conventional name
formed by specialization. In LMS, we represent it as (TREE
APPLE). Note that no strong distinction is made between
conventional names that are compound words and those that are phrases
in English. Compare "fire hydrant," (HYDRANT FIRE), and
"fireman", (MAN FIRE).
(TREE TALL), and "the apple tree in my yard",
(((TREE APPLE) THE) (IN (YARD MY))).7 The difference between identifying and
designating is often slight. In designating, we assume that the hearer
already knows the concept, whereas in identifying, we ask him to come
to know it from what he knows of its components and whatever else we
may later tell him. Thus, a "shoe tree," which we might initially
accept as an identifying compound without a conventional designation,
may come to designate a concept if shoe trees become a popular
consumer item. Just as compound words develop from conventional names
that are phrases, the latter develop from identifying phrases.
(BOOK -S), the -S is a grammatical
marking for plural on the base concept BOOK. In
"hitting," (HIT -ING), and "to jump," (JUMP
TO), the -ING and TO play a similar
role. This form of marking is called inflection. In LMS, inflection is
expressed by specializing the concept to be inflected by the affix (or
other marker).
(SIZE
APPLE).
If we compare the red door and the barking dog, on the one hand (junction) and on the other the door is red and the dog barks or the dog is barking (nexus), we find that the former kind is more rigid or stiff, and the latter more pliable; there is, as it were, more life in it. A junction is like a picture, a nexus is like a drama or process. In a nexus, nothing new is added to the conception contained in the primary; the difference between that and a junction is seen clearly by comparing, e.g.The blue dress is the oldest.
The oldest dress is blue.
A dancing woman charms.
A charming woman dances.
In our terms, a junction identifies or designates. A nexus, or predication, makes a statement and depends on interpretation for its meaning.
In LMS, we introduce a new notation to express predication:
subject / predicate. For example,
Jespersen's sentence "the oldest dress is blue" becomes ((DRESS
OLDEST) THE) / BLUE. For uniformity of representation and
implementational convenience, however, we will implement predication
in LMS using specialization by adopting the following convention: The
predication A / B will be implemented as ((B NEXUS)
A).
GENERAL for (OFFICER GENERAL),
CAPITAL for (LETTER CAPITAL), and
EMPTY for (CONTAINER EMPTY). In each of
these cases, the specializer in context names the whole concept. We
shall encounter more general uses of naming below.
Our treatment of specialization as outlined above is inadequate for
some subtler issues of representation. Although we have identified
several uses of compound formation in English communication, we have
represented them all by the same specialization operation. We form, in
a completely similar manner, compound phrases like "the dog,"
(DOG THE), "sheep dog," (DOG SHEEP), "small
dog," (DOG SMALL), and "dog In the yard." (DOG (IN
(YARD THE))). For these examples, no problems arise because we
can recapture from the specializer itself what kind of compound we
have formed. But that will not generally be the case, as we shall see
below. In this section, we introduce seven distinct kinds of
specialization to enrich our representation scheme.
The English phrase "fat man" is ambiguous. In its common meaning,
it stands for a man who is overweight to some degree. The same phrase,
however, also describes a professional circus performer of great
girth, with whom we associate characteristic forms of dress, behavior,
etc. In terms of the modes of communication listed above, we are
either (1) designating the circus performer by his conventional name
or (2) identifying the man who is overweight by his genus and a
distinguishing characteristic.8 OWL is
unique in that we make a procedural distinction between these two
senses of "fat man." In the first case, "fat" is combined with "man"
to identify a pattern in memory, and then that pattern is used to find
the referent. In the second case, "man" alone is used to find a
pattern in memory, and then items which match this pattern are further
checked to see if they pass the pattern designated by "fat." We could
imagine a skinny fat man only in the first sense, as referring to the
circus performer. But our representational scheme, as presented so
far, offers only (MAN FAT) for "fat man," and fails to
distinguish the two senses we have discussed.
To preserve the desired distinction between these readings of "fat
man," we will mark every specialization with its meta-type,
which indicates the relation between the concept and its genus.9 We will represent our overweight man by a
restrictive specialization, (MAN*R FAT). A
restriction (A*R B) may always be paraphrased as
"an A which is B," e.g., "a man who is fat," and a restriction always
represents a concept which is a kind of its genus with the additional
attribute which is its specializer. Note that a tall fat man,
((MAN*R FAT)*R TALL) is not the same as a fat tall man,
((MAN*R TALL)*R FAT), either in real life or in
conceptual memory. In a stereotype, (A*T B), the
specializer has some close relation to the genus but is not
necessarily a property of it. Consider not just our circus performer,
(MAN*T FAT), but also (HYDRANT*T FIRE),
where the relation between "fire" and "hydrant" is a complex one: "a
hydrant which is a source of water with which one can put out a fire."
The seven OWL meta-types and their notational suffixes are:
*R restriction
*T stereotype
*S species
*I instance
*A aspect
*X inflection
*P partitive
(A*S B) represents a subspecies of
A, where B is often just a symbol. This
represents a Linnaean classification system in which we assume that
different subspecies of A form mutually exclusive
categories. This is a powerful tool for database search. (A*I
B) represents an instance of A. Instances, as
species, are mutually exclusive.10 We
thus provide a distinction between classes and individuals by
distinguishing instances from species.
An aspect specialization (C*A B) is a kind of its
genus C, which is closely associated with its specializer
B. For example, "height of John," (HEIGHT*A
JOHN) and "John's leg," (LEG*A JOHN). Aspects also
play the traditional role of programming language variables. For
example, if we have a recipe for pancakes which calls for one egg,
that egg will be represented by (EGG*A (RECIPE*T
PANCAKE)).
An inflection, (A*X B), is used to specify a
grammatical or interpretive aspect. It has the unusual behavior that
it inherits properties not only from its genus, as all other
specialization types do, but also from its specializer. In fact,
properties inherited from the specializer override any inherited from
the genus. For example, "books," (BOOK*X -S), is plural
even though BOOK is singular, because -S
carries the plural property.
The partitive, (A*P B), is like a semantic version of
inflection. The partitive inherits properties from both its genus and
specializer, where context determines the appropriate
interpretation. Thus, one may first open and then eat a can of beans,
first opening the can and then eating the beans.
The above is a short sketch of our approach to representation. A much more complete treatment will be found In (Martin 1977].
To translate from strings of English words to their representation, we use an augmented transition network parser based on (Woods 1970]. The OWL parser uses no registers but maintains a constituent stack of concepts with each phrase for which a transition network (TN) is being followed. On every arc is an OWL concept which must be matched for that transition to apply and a set of combining functions which manipulate the matching concept and constituent stack.
It is the task of the combining functions to compose OWL concepts representing parts of a phrase into the concept representing the whole phrase. The role of the TN is to invoke the combining functions in the appropriate sequence. The parser operates nondeterministically (via backtracking). Failure leading to backup may occur either because the input string fails to meet word-order constraints (i.e., no match can be found for any arc from a non-terminal node of a TN) or because a combining function rejects a proposed phrase. The conceptual memory contains (expressed via attachment) strictly enforced constraints on case slots of all grammatical concepts. Using these constraints, the combining functions control all compositions such as adjectival and adverbial modification and case assignment for verb phrases. The word-order constraints of the TN's plus the concept-formation constraints in the conceptual memory (as they are used by the combining functions) thus express our grammar.
Two mechanisms of special interest should be mentioned: the use of
naming to postpone the introduction of ambiguity, and bidding. Because
many English words and phrases have alternate interpretations in LMS
(e.g., our "fat man" example), if we were to split our computation
nondeterministically every time alternative interpretations of a
phrase were available, we would spend a lot of processing effort
carrying all those interpretations along until all but one could be
eliminated. Further, if more than one interpretation succeeded and the
sentence parsed ambiguously, we would have a difficult task localizing
the cause of the ambiguity. To avoid these problems, we take a "wait
and see" approach11 and try not to choose
the appropriate Interpretation until some further constraint forces
that choice. Postponing the choice Is accomplished by use of the
namIng mechanism introduced above. In our "fat man" example, we say
that conventionally we will form the restriction (MAN*R
FAT) as the interpretation of the phrase and we will have in
the knowledge base an indication that (MAN*R FAT) names
(MAN*T FAT). In this case, the distinction may never have
to be drawn during parsing, since no grammatical decisions will depend
on It, and it will be some later step of reasoning in the system that
may have to choose the "circus performer" interpretation.
In a typical situation where grammatical distinctions arise early
in parsing, we take a slightly different approach from the previous
example. The word "drinks" is either the plural of the noun "drink,"
as in "We had a few drinks," or the third person singular of the verb
"drink," as in "Joe drinks beer at dinnertime." Here, rather than
choosing one of these as a primary interpretation, we create the
neutral (DRINK*X -S) and say that it names both
(DRINK*X PLURAL-NOUN) and (DRINK*X
THIRD-PERSON-SINGULAR-VERB). To make this scheme work, every
combining function must succeed not only when the concepts given to it
may be directly combined but also when any concepts named by the given
ones may be combined. Matching of concepts on TN arcs is similarly
augmented. Further, rules like the above for "drink" generalize, and
OWL encodes those generalizations rather than specific naming rules
for each.12
Bidding is another mechanism for deferring a choice among alternatives and avoiding undue nondeterminism. Its application is best seen when considering the attachment of prepositional phrases. For example, in "I rode along the highway in my limousine," we may eliminate "the highway in my limousine" as implausible and attach the prepositional phrase to the predicate (or predication). By contrast, in "I liked the phone in my limousine," the prepositional phrase clearly belongs with "phone." We cannot always make such a definitive judgment: "I saw the man beside our house" places either me or the man beside the house. From further context, the ambiguity may be resolved: "As I approached, I saw the man beside our house." We treat this problem by suspending a path in parsing at a point where it is about to take an arc transition for a prepositional phrase until all possible paths leading to taking such a transition for that same phrase are identified. Then, a conflict-resolving routine is called to permit any number of the possible interpretations to proceed. That routine will, in general, invoke the Interpreter to try to decide which interpretation(s) are best. Its success will depend on the sophistication of world knowledge in the conceptual memory and on the existence of appropriate strategies available to the Interpreter to apply that knowledge. A more specific mechanism which similarly addresses the problem of "selective modifier placement" is presented in [Woods 1973]. We have not yet made any significant use of this bidding strategy.
We have implemented an initial version of an Interpreter for OWL, which is the basis of the system's ability to reason. It is a large program with many interesting capabilitIes, of which we will here describe only the central ones. Sunguroff (1976) describes the implementation details of the current version, Brown (1977) Is concerned with use of the Interpreter for dialog and the handling of failure, Long (1977) gives another view of the Interpreter's use for automatic programming, and Swartout (1977) discusses the Interpreter's record-keeping and updating capabilities and their relation to explaining program behavior.
So far, we have interpreted OWL concepts as static entities, mere translations of English phrases. The system's action when given the sentence "Prescribe an appropriate dosage of Digitalis for Mr. Jones" cannot be merely to translate that sentence into its internal representation and then stop. But how is it to know what the procedural meaning of some sentence is?
If an OWL concept has a METHOD aspect, then it is
called a PLAN and is something which the Interpreter can
carry out. When the Interpreter is called (its argument is the call),
it performs the following steps:
PREREQUISITE aspect, it checks
if it is already TRUE and if not, then it tries to make
it true. This subgoal step of course once again uses the Interpreter.
METHOD, either in
parallel or in sequence, whichever is specified.
We attempt always to use the Interpreter to solve subproblems of an
initial problem so that the general matching and reasoning resources
we build up will be available at all levels. For example, if
X is a prerequisite which is not yet satisfied, we merely
call the Interpreter with the call (GET*T X). Classical
goal-directed behavior can be achieved by use of the
PRINCIPAL-RESULT case on a plan, which identifies the
teleological goal of the plan. Then, if a GET is unable
to find a plan by its upward search of the concept tree, it may search
for a matching principal result and select the plan which promises
that result. One other important aspect of the Interpreter is that
after every step of interpretation, it dispatches to its next step
through the main top-level loop. There, failure-handling and
advice-giving procedures may always be invoked to redirect the course
of computation by "backing off" from unproductive lines (if they can
be recognized).
We are continuing to refine our understanding of the representation or English phrases in the formal notation of OWL and the use of a complex interpreter which works within that formalism to perform all reasoning tasks which arise in language processing and various application areas.
1. We limit ourselves to "left-hemisphere knowledge," which does not include visual skills or manipulative skills where local muscle/nerve training is an essential component. Thus, our domains are restricted to reasoning tasks where the necessary data about a problem can be acquired verbally; e.g., medical diagnosis and treatment of the type which could be done by consultation over the telephone (probably not, for example, diagnosis of skin disease, where visual inspection is a critical skill), automatic program writing, question answering.
2. The general implication of is-a or is
a kind of (AKO) links is that "something" (properties, features,
place of classification, ways to treat, etc.) is inherited by
C from B. We will define this more
precisely later.
3. For example, we may represent "dog house" as
(HOUSE DOG) and "dog tail" as (TAIL DOG),
and although both concepts are specialized by DOG, they
are clearly different.
4. An intermediate concept in the taxonomy, such as
(TAIL ANIMAL) in our example, is automatically created by
LMS whenever more than one concept may be classified under it. Thus,
the generalizer of a concept, and hence the number of times that we
need to move from a concept to its generalizer in order to reach its
genus, will depend dynamically on what other concepts are in the
taxonomy.
5. It is not merely the representation of truth that is at issue here. A similar treatmnet is necessary for supposition, hypothesis, "possible futures," and in fact all the fundamental knowledge on the basis of which OWL operates. Of course the effect of the meta-level statements that we allow could alternatively be introduced by suitable conventions for the Interpreter. For example, we could adopt the convention that any statement about which no qualifying information is known is true. We prefer, however, to make such a convention part of the Interpreter and not part of the semantics of LMS.
6. "APPLE" is the LMS notation for the
symbol "apple". The concept (FRUIT "APPLE") is LMS's
notation for the English concept apple.
7. Some linguists might feel that this phrase
should have a different structure, such as (((TREE APPLE) (IN
(YARD MY))) THE). We do not claim to have the final answer to
all such structural questions, but our formalism allows us to
capitalize on whatever insights linguists may have. Where structures
are in dispute, we have chosen what seems best to us.
8. In spoken language, the compound representing the conventional name is spoken almost as if it were the compound word "fatman." This additional clue is not available to us via written language.
9. We are introducing a minor inconsistency here,
because we change the meaning of "genus" somewhat. By the rules of
LMS, the genus of the concept (A*R B) is
A*R, yet we will refer here to A, the
concept's linguistic genus, as its genus.
10. Some systems further divide instances into
manifestations: e.g., "the young Churchill." We would handle this as
(CHURCHILL*R YOUNG), where CHURCHILL is
(MAN*I "CHURCHILL").
11. This technique is motivated by (Waltz 1972) and also applied in a parser by Marcus (1975).
12. These naming generalizations arc called productive naming rules. They arc applied by the normal inheritance mechanism of LMS, so of course they may be overridden by more specific information in any particular case.
13. This is a very important idea. With it, we can embed completely specific plans to solve any problems which we know will arise often and will be critical to the system's performance. We also use it to express plans when their choice is dictated not by a reasoned choice but by convention in the application area. If a specific plan is unavailable, slightly more general plans will he attempted, and only if all such plans arc found inapplicable will the system resort to some general deductive scheme. We have noted that only when a great majority of specific plans for a domain is available will the system's performance be at an "expert" level. This agrees with our observations that human experts seem to have large portions of their ordinary professional behavior "precompiled" into fixed routines.
[Bosyj 1976] Bosyj, Michael, A Program for the Design of Procurement Systems. Cambridge, Mass.: MIT Laboratory for Computer Science TR-160
[Brown 1977] Brown, Gretchen P., "A System to Process Dialogue A Progress Report," MIT Laboratory for Computer Science TM-79
[Hawkinson 1975] Hawkinson, Lowell B., "The Representation of Concepts in OWL," Proceedings of the Fourth International Joint Conference on Artificial Intelligence
[Jespersen 1933] Jespersen, Otto, Essentials of English Grammar, University of Alabama Press, 1964
[Long 1977] Long, William J., "A Program Writer," Ph. D. Dissertation, MIT (in preparation)
[Marcus 1975] Marcus, Mitchell, "Diagnosis as a Notion of Grammar", In Shank, Nash Webber, eds., Preprints from the Workshop in Theoretical Issues In Natural Language Understanding, 1975
[Martin 1977] Martin, William A., "A Theory of English Grammar" (in preparation)
[Rahmstorf 1977] Rahmstorf, G. and M. Ferguson, eds., Natural Language for Interaction with Data Bases, Proceedings of an International Workshop held at IIASA Laxenburg, Jan 10-14, 1977. Laxenburg 1977, IIASA.
[Swartout 1977] Swartout, William R., "A Digitalis Therapy Advisor with Explanations," MIT Laboratory for Computer Science TR-176
[Sunguroff 1976] Sunguroff, Alex, "OWL Interpreter Reference Manual," MIT Laboratory for Computer Science, Automatic Programming Group (unpublished internal documentation)
[Woods 1970] Woods, W. A., "Transition Network Grammars for Natural Language Analysis," Communications of the ACM, 13, 10 (October 1970)
[Woods 1973] Woods, W. A., "An Experimental Parsing System for Transition Network Grammars," in Rustin, R., ed., Natural Language Processing, New York: Algorithmics Press.
[Waltz 1972] Waltz, David L., "Generating Semantic Descriptions from Drawings of Scenes with Shadows," MIT Artificial Intelligence Laboratory TR-271
This paper was reconstructed in HTML in 2000 by Peter Szolovits, as part of his on-line publications archive.