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SOME EXPERT SYSTEM NEED COMMON
Computer Science Department
Stanford, CA 94305
An expert system is a computer program intended to embody the knowl-
edge and ability of an expert in a certain domain. The ideas behind them and
several examples have been described in other lectures in this symposium.
Their performance in their specialized domains are often very impressive.
Nevertheless, hardly any of them have certain common sense knowledge and
ability possessed by any non-feeble-minded human. This lack makes them
“brittle”. By this is meant that they are diﬃcult to extend beyond the scope
originally contemplated by their designers, and they usually don’t recognize
their own limitations. Many important applications will require common
sense abilities. The object of this lecture is to describe common sense abili-
ties and the problems that require them.
Common sense facts and methods are only very partially understood to-
day, and extending this understanding is the key problem facing artiﬁcial
This isn’t exactly a new point of view. I have been advocating “Com-
puter Programs with Common Sense”since I wrote a paper with that title in
1958. Studying common sense capability has sometimes been popular and
sometimes unpopular among AI researchers. At present it’s popular, per-
haps because new AI knowledge oﬀers new hope of progress. Certainly AI
researchers today know a lot more about what common sense is than I knew
in 1958 — or in 1969 when I wrote another paper on the subject. However,
expressing common sense knowledge in formal terms has proved very diﬃcult,
and the number of scientists working in the area is still far too small.
One of the best known expert systems is MYCIN (Shortliﬀe 1976; Davis,
Buchanan and Shortliﬀe 1977), a program for advising physicians on treating
bacterial infections of the blood and meningitis.
It does reasonably well
without common sense, provided the user has common sense and understands
the program’s limitations.
MYCIN conducts a question and answer dialog. After asking basic facts
about the patient such as name, sex and age, MYCIN asks about suspected
bacterial organisms, suspected sites of infection, the presence of speciﬁc
symptoms (e.g. fever, headache) relevant to diagnosis, the outcome of labo-
ratory tests, and some others. It then recommends a certain course of antibi-
otics. While the dialog is in English, MYCIN avoids having to understand
freely written English by controlling the dialog. It outputs sentences, but the
user types only single words or standard phrases. Its major innovations over
many previous expert systems were that it uses measures of uncertainty (not
probabilities) for its diagnoses and the fact that it is prepared to explain its
reasoning to the physician, so he can decide whether to accept it.
Our discussion of MYCIN begins with its ontology. The ontology of a
program is the set of entities that its variables range over. Essentially this is
what it can have information about.
MYCIN’s ontology includes bacteria, symptoms, tests, possible sites of in-
fection, antibiotics and treatments. Doctors, hospitals, illness and death are
absent. Even patients are not really part of the ontology, although MYCIN
asks for many facts about the speciﬁc patient. This is because patients aren’t
values of variables, and MYCIN never compares the infections of two diﬀer-
ent patients. It would therefore be diﬃcult to modify MYCIN to learn from
MYCIN’s program, written in a general scheme called EMYCIN, is a so-
called production system. A production system is a collection of rules, each
of which has two parts — a pattern part and an action part. When a rule
is activated, MYCIN tests whether the pattern part matches the database.
If so this results in the variables in the pattern being matched to whatever
entities are required for the match of the database. If not the pattern fails
and MYCIN tries another. If the match is successful, then MYCIN performs
the action part of the pattern using the values of the variables determined
by the pattern part. The whole process of questioning and recommending is
built up out of productions.
The production formalism turned out to be suitable for representing a
large amount of information about the diagnosis and treatment of bacterial
infections. When MYCIN is used in its intended manner it scores better than
medical students or interns or practicing physicians and on a par with experts
in bacterial diseases when the latter are asked to perform in the same way.
However, MYCIN has not been put into production use, and the reasons given
by experts in the area varied when I asked whether it would be appropriate to
sell MYCIN cassettes to doctors wanting to put it on their micro-computers.
Some said it would be ok if there were a means of keeping MYCIN’s database
current with new discoveries in the ﬁeld, i.e. with new tests, new theories,
new diagnoses and new antibiotics. For example, MYCIN would have to
be told about Legionnaire’s disease and the associated Legionnella bacteria
which became understood only after MYCIN was ﬁnished. (MYCIN is very
stubborn about new bacteria, and simply replies “unrecognized response”.)
Others say that MYCIN is not even close to usable except experimen-
tally, because it doesn’t know its own limitations. I suppose this is partly a
question of whether the doctor using MYCIN is trusted to understand the
documentation about its limitations. Programmers always develop the idea
that the users of their programs are idiots, so the opinion that doctors aren’t
smart enough not to be misled by MYCIN’s limitations may be at least partly
a consequence of this ideology.
An example of MYCIN not knowing its limitations can be excited by
telling MYCIN that the patient has Cholerae Vibrio in his intestines. MYCIN
will cheerfully recommend two weeks of tetracycline and nothing else. Pre-
sumably this would indeed kill the bacteria, but most likely the patient will
be dead of cholera long before that. However, the physician will presumably
know that the diarrhea has to be treated and look elsewhere for how to do
On the other hand it may be really true that some measure of common
sense is required for usefulness even in this narrow domain. We’ll list some
areas of common sense knowledge and reasoning ability and also apply the
criteria to MYCIN and other hypothetical programs operating in MYCIN’s
1 WHAT IS COMMON SENSE?
Understanding common sense capability is now a hot area of research in
artiﬁcial intelligence, but there is not yet any consensus. We will try to
divide common sense capability into common sense knowledge and common
sense reasoning, but even this cannot be made ﬁrm. Namely, what one man
builds as a reasoning method into his program, another can express as a fact
using a richer ontology. However, the latter can have problems in handling
in a good way the generality he has introduced.
2 COMMON SENSE KNOWLEDGE
We shall discuss various areas of common sense knowledge.
and for a program to plan intelligently, it must be able to determine the
eﬀects of its own actions.
Consider the MYCIN domain as an example. The situation with which
MYCIN deals includes the doctor, the patient and the illness. Since MYCIN’s
actions are advice to the doctor, full planning would have to include infor-
mation about the eﬀects of MYCIN’s output on what the doctor will do.
Since MYCIN doesn’t know about the doctor, it might plan the eﬀects of
the course of treatment on the patient. However, it doesn’t do this either.
Its rules give the recommended treatment as a function of the information
elicited about the patient, but MYCIN makes no prognosis of the eﬀects of
the treatment. Of course, the doctors who provided the information built
into MYCIN considered the eﬀects of the treatments.
Ignoring prognosis is possible because of the speciﬁc narrow domain in
which MYCIN operates. Suppose, for example, a certain antibiotic had
the precondition for its usefulness that the patient not have a fever. Then
MYCIN might have to make a plan for getting rid of the patient’s fever and
verifying that it was gone as a part of the plan for using the antibiotic. In
other domains, expert systems and other AI programs have to make plans,
but MYCIN doesn’t. Perhaps if I knew more about bacterial diseases, I
would conclude that their treatment sometimes really does require planning
and that lack of planning ability limits MYCIN’s utility.
The fact that MYCIN doesn’t give a prognosis is certainly a limitation.
For example, MYCIN cannot be asked on behalf of the patient or the admin-
istration of the hospital when the patient is likely to be ready to go home.
The doctor who uses MYCIN must do that part of the work himself. More-
over, MYCIN cannot answer a question about a hypothetical treatment, e.g.
“What will happen if I give this patient penicillin?” or even “What bad
things might happen if I give this patient penicillin?”.
I know about give the eﬀects of an event in a situation by describing a new
situation that results from the event. This is often enough, but it doesn’t
cover the important case of concurrent events and actions. For example, if
a patient has cholera, while the antibiotic is killing the cholera bacteria, the
damage to his intestines is causing loss of ﬂuids that are likely to be fatal.
Inventing a formalism that will conveniently express people’s common sense
knowledge about concurrent events is a major unsolved problem of AI.
facts about this are diﬃcult to express but are probably not important in
the MYCIN example. A major diﬃculty is in handling the kind of partial
knowledge people ordinarily have. I can see part of the front of a person in
the audience, and my idea of his shape uses this information to approximate
his total shape. Thus I don’t expect him to stick out two feet in back even
though I can’t see that he doesn’t. However, my idea of the shape of his back
is less deﬁnite than that of the parts I can see.
is recorded in the issue of the International Airline Guide current for the
proposed ﬂight day. Travel agents know how to book airline ﬂights and can
compute what they cost. An advanced MYCIN might need to reason that Dr.
Smith knows about cholera, because he is a specialist in tropical medicine.
goals, likes and dislikes, intentions and abilities. An advanced MYCIN might
need to know that a patient won’t take a bad tasting medicine unless he is
convinced of its necessity.
spill the glass of water on the podium, everyone knows that the glass will
break and the water will spill. Everyone knows that this will take a fraction
of a second and that the water will not splash even ten feet. However, this
information is not obtained by using the formula for a falling body or the
Navier-Stokes equations governing ﬂuid ﬂow. We don’t have the input data
for the equations, most of us don’t know them, and we couldn’t integrate
them fast enough to decide whether to jump out of the way. This common
sense physics is contiguous with scientiﬁc physics. In fact scientiﬁc physics is
imbedded in common sense physics, because it is common sense physics that
tells us what the equation s = 0.5gt2 means. If MYCIN were extended to be
a robot physician it would have to know common sense physics and maybe
also some scientiﬁc physics.
It is doubtful that the facts of the common sense world can be represented
adequately by production rules. Consider the fact that when two objects
collide they often make a noise. This fact can be used to make a noise,
to avoid making a noise, to explain a noise or to explain the absence of a
noise. It can also be used in speciﬁc situations involving a noise but also to
understand general phenomena, e.g. should an intruder step on the gravel,
the dog will hear it and bark. A production rule embodies a fact only as part
of a speciﬁc procedure. Typically they match facts about speciﬁc objects,
e.g. a speciﬁc bacterium, against a general rule and get a new fact about
Much present AI research concerns how to represent facts in ways that
permit them to be used for a wide variety of purposes.
3 COMMON SENSE REASONING
Our ability to use common sense knowledge depends on being able to do
common sense reasoning.
Much artiﬁcial intelligence inference is not designed to use directly the
rules of inference of any of the well known systems of mathematical logic.
There is often no clear separation in the program between determining what
inferences are correct and the strategy for ﬁnding the inferences required to
solve the problem at hand. Nevertheless, the logical system usually corre-
sponds to a subset of ﬁrst order logic. Systems provide for inferring a fact
about one or two particular objects from other facts about these objects and
a general rule containing variables. Most expert systems, including MYCIN,
never infer general statements, i.e. quantiﬁed formulas.
Human reasoning also involves obtaining facts by observation of the world,
and computer programs also do this. Robert Filman did an interesting thesis
on observation in a chess world where many facts that could be obtained by
deduction are in fact obtained by observation. MYCIN’s doesn’t require this,
but our hypothetical robot physician would have to draw conclusions from a
patient’s appearance, and computer vision is not ready for it.
An important new development in AI (since the middle 1970s) is the
formalization of nonmonotonic reasoning.
Deductive reasoning in mathematical logic has the following property —
called monotonicity by analogy with similar mathematical concepts. Sup-
pose we have a set of assumptions from which follow certain conclusions.
Now suppose we add additional assumptions. There may be some new con-
clusions, but every sentence that was a deductive consequence of the original
hypotheses is still a consequence of the enlarged set.
Ordinary human reasoning does not share this monotonicity property. If
you know that I have a car, you may conclude that it is a good idea to ask
me for a ride. If you then learn that my car is being ﬁxed (which does not
contradict what you knew before), you no longer conclude that you can get
a ride. If you now learn that the car will be out in half an hour you reverse
Several artiﬁcial intelligence researchers, for example Marvin Minsky (1974)have pointed out that intelligent computer programs will have to reason non-
monotonically. Some concluded that therefore logic is not an appropriate
However, it has turned out that deduction in mathematical logic can be
supplemented by additional modes of nonmonotonic reasoning, which are just
as formal as deduction and just as susceptible to mathematical study and
computer implementation. Formalized nonmonotonic reasoning turns out to
give certain rules of conjecture rather than rules of inference — their conclu-
sion are appropriate, but may be disconﬁrmed when more facts are obtained.
One such method is circumscription, described in (McCarthy 1980).
A mathematical description of circumscription is beyond the scope of
this lecture, but the general idea is straightforward. We have a property
applicable to objects or a relation applicable to pairs or triplets, etc. of
objects. This property or relation is constrained by some sentences taken
as assumptions, but there is still some freedom left. Circumscription further
constrains the property or relation by requiring it to be true of a minimal set
As an example, consider representing the facts about whether an object
can ﬂy in a database of common sense knowledge. We could try to provide
axioms that will determine whether each kind of object can ﬂy, but this
would make the database very large. Circumscription allows us to express
the assumption that only those objects can ﬂy for which there is a positive
statement about it. Thus there will be positive statements that birds and
airplanes can ﬂy and no statement that camels can ﬂy. Since we don’t include
negative statements in the database, we could provide for ﬂying camels, if
there were any, by adding statements without removing existing statements.
This much is often done by a simpler method — the closed world assumption
discussed by Raymond Reiter. However, we also have exceptions to the
general statement that birds can ﬂy. For example, penguins, ostriches and
birds with certain feathers removed can’t ﬂy. Moreover, more exceptions may
be found and even exceptions to the exceptions. Circumscription allows us
to make the known exceptions and to provide for additional exceptions to be
added later — again without changing existing statements.
Nonmonotonic reasoning also seems to be involved in human communi-
cation. Suppose I hire you to build me a bird cage, and you build it without
a top, and I refuse to pay on the grounds that my bird might ﬂy away. A
judge will side with me. On the other hand suppose you build it with a top,
and I refuse to pay full price on the grounds that my bird is a penguin, and
the top is a waste. Unless I told you that my bird couldn’t ﬂy, the judge will
side with you. We can therefore regard it as a communication convention
that if a bird can ﬂy the fact need not be mentioned, but if the bird can’t ﬂy
and it is relevant, then the fact must be mentioned.
Davis, Randall; Buchanan, Bruce; and Shortliﬀe, Edward (1977). Production
Rules as a Representation for a Knowledge-Based Consultation Program,
Artiﬁcial Intelligence, Volume 8, Number 1, February.
McCarthy, John (1960). Programs with Common Sense, Proceedings of the
Teddington Conference on the Mechanization of Thought Processes, London:
Her Majesty’s Stationery Oﬃce. (Reprinted in this volume, pp. 000–000).
McCarthy, John and Patrick Hayes (1969). Some Philosophical Problems
from the Standpoint of Artiﬁcial Intelligence, in B. Meltzer and D. Michie
(eds), Machine Intelligence 4, Edinburgh University.
(Reprinted in B. L.
Webber and N. J. Nilsson (eds.), Readings in Artiﬁcial Intelligence, Tioga,
1981, pp. 431–450; also in M. J. Ginsberg (ed.), Readings in Nonmonotonic
Reasoning, Morgan Kaufmann, 1987, pp. 26–45; also in this volume, pp.
McCarthy, John (1980). Circumscription — A Form of Nonmonotonic Rea-
soning, Artiﬁcial Intelligence, Volume 13, Numbers 1,2. (Reprinted in B. L.
Webber and N. J. Nilsson (eds.), Readings in Artiﬁcial Intelligence, Tioga,
1981, pp. 466–472; also in M. J. Ginsberg (ed.), Readings in Nonmonotonic
Reasoning, Morgan Kaufmann, 1987, pp. 145–152; also in this volume, pp.
Minsky, Marvin (1974). A Framework for Representing Knowledge, M.I.T.
AI Memo 252.
Shortliﬀe, Edward H. (1976). Computer-Based Medical Consultations: MYCIN,American Elsevier, New York, NY.
ANSWERS TO QUESTIONS
DISCUSSION OF THE PAPER
QUESTION: You said the programs need common sense, but that’s like
saying, If I could ﬂy I wouldn’t have to pay Eastern Airliness $44 to haul me
up here from Washington. So if the programs indeed need common sense,
how do we go about it? Isn’t that the point of the argument?
DR. MCCARTHY: I could have made this a defensive talk about artiﬁcial
intelligence, but I chose to emphasize the problems that have been identiﬁed
rather than the progress that has been made in solving them. Let me remind
you that I have argued that the need for common sense is not a truism. Many
useful things can be done without it, e.g. MYCIN and also chess programs.
QUESTION: There seemed to be a strong element in your talk about common
sense, and even humans developing it, emphasizing an experiential compo-
nent — particularly when you were giving your example of dropping a glass
of water. I’m wondering whether the development of these programs is going
to take similar amounts of time. Are you going to have to have them go
through the sets of experiences and be evaluated? Is there work going on in
terms of speeding up the process or is it going to take 20 years for a program
from the time you’ve put in its initial state to work up to where it has a
decent amount of common sense?
DR. MCCARTHY: Consider your 20 years. If anyone had known in 1963
how to make a program learn from its experience to do what a human does
after 20 years, they might have done it, and it might be pretty smart by now.
Already in 1958 there had been work on programs that learn from experi-
ence. However, all they could learn was to set optimal values of numerical
parameters in the program, and they were quite limited in their ability to
do that. Arthur Samuel’s checker program learned optimal values for its
parameters, but the problem was that certain kinds of desired behavior did
not correspond to any setting of the parameters, because it depended on the
recognition of a certain kind of strategic situation. Thus the ﬁrst prerequisite
for a program to be able to learn something is that it be able to represent
internally the desired modiﬁcation of behavior. Simple changes in behavior
must have simple representations. Turing’s universality theory convinces us
that arbitrary behaviors can be represented, but they don’t tell us how to
represent them in such a way that a small change in behavior is a small
change in representation. Present methods of changing programs amount to
education by brain surgery.
QUESTION: I would ask you a question about programs needing common
sense in a slightly diﬀerent way, and I want to use the MYCIN program as
There are three actors there — the program, the physician, and the pa-
tient. Taking as a criterion the safety of the patient, I submit that you need
at least two of these three actors to have common sense.
For example if (and sometimes this is the case) one only were suﬃcient,
it would have to be the patient because if the program didn’t use common
sense and the physician didn’t use common sense, the patient would have to
have common sense and just leave. But usually, if the program had common
sense built in and the physician had common sense but the patient didn’t, it
really might not matter because the patient would do what he or she wants
to do anyway.
Let me take another possibility. If only the program has common sense
and neither the physician nor the patient has common sense, then in the long
run the program also will not use the common sense. What I want to say is
that these issues of common sense must be looked at in this kind of frame of
DR. MCCARTHY: In the use of MYCIN, the physician is supposed to supply
the common sense. The question is whether the program must also have
common sense, and I would say that the answer is not clear in the MYCIN
case. Purely computational programs don’t require common sense, and none
of the present chess programs have any. On the other hand, it seems clear
that many other kinds of programs require common sense to be useful at all.
/@steam.stanford.edu:/u/ftp/jmc/someneed.tex: begun 1996 May 12, latexed 1996 May 12 at 1:24 p.m.