Extinguished philosophies lie about the cradle of every science as the

strangled snakes beside that of Hercules. - adapted from T. H. Huxley

WHAT IS ARTIFICIAL INTELLIGENCE?

John McCarthy

Computer Science Department

Stanford University

Stanford, CA 94305

jmc@cs.stanford.edu

http://www-formal.stanford.edu/jmc/

2007 Nov 12, 2:05 a.m.

Revised November 12, 2007:

Abstract

This article for the layman answers basic questions about artificial

intelligence. The opinions expressed here are not all consensus opinion

among researchers in AI.

1 Basic Questions

Q. What is artificial intelligence?

A. It is the science and engineering of making intelligent machines, es-

pecially intelligent computer programs. It is related to the similar task of

using computers to understand human intelligence, but AI does not have to

confine itself to methods that are biologically observable.

Q. Yes, but what is intelligence?

A. Intelligence is the computational part of the ability to achieve goals in

the world. Varying kinds and degrees of intelligence occur in people, many

animals and some machines.

Q. Isn’t there a solid definition of intelligence that doesn’t depend on

relating it to human intelligence?

A. Not yet. The problem is that we cannot yet characterize in general

what kinds of computational procedures we want to call intelligent. We

understand some of the mechanisms of intelligence and not others.

Q. Is intelligence a single thing so that one can ask a yes or no question

“Is this machine intelligent or not?”?

A. No. Intelligence involves mechanisms, and AI research has discovered

how to make computers carry out some of them and not others. If doing a

task requires only mechanisms that are well understood today, computer pro-

grams can give very impressive performances on these tasks. Such programs

should be considered “somewhat intelligent”.

Q. Isn’t AI about simulating human intelligence?

A. Sometimes but not always or even usually. On the one hand, we can

learn something about how to make machines solve problems by observing

other people or just by observing our own methods. On the other hand, most

work in AI involves studying the problems the world presents to intelligence

rather than studying people or animals. AI researchers are free to use meth-

ods that are not observed in people or that involve much more computing

than people can do.

Q. What about IQ? Do computer programs have IQs?

A. No. IQ is based on the rates at which intelligence develops in children.

It is the ratio of the age at which a child normally makes a certain score

to the child’s age. The scale is extended to adults in a suitable way.

IQ

correlates well with various measures of success or failure in life, but making

computers that can score high on IQ tests would be weakly correlated with

their usefulness. For example, the ability of a child to repeat back a long

sequence of digits correlates well with other intellectual abilities, perhaps

because it measures how much information the child can compute with at

once. However, “digit span” is trivial for even extremely limited computers.

However, some of the problems on IQ tests are useful challenges for AI.

Q. What about other comparisons between human and computer intelli-

gence?

Arthur R. Jensen [Jen98], a leading researcher in human intelligence,

suggests “as a heuristic hypothesis” that all normal humans have the same

intellectual mechanisms and that differences in intelligence are related to

“quantitative biochemical and physiological conditions”. I see them as speed,

short term memory, and the ability to form accurate and retrievable long term

memories.

Whether or not Jensen is right about human intelligence, the situation in

AI today is the reverse.

Computer programs have plenty of speed and memory but their abilities

correspond to the intellectual mechanisms that program designers understand

well enough to put in programs. Some abilities that children normally don’t

develop till they are teenagers may be in, and some abilities possessed by

two year olds are still out. The matter is further complicated by the fact

that the cognitive sciences still have not succeeded in determining exactly

what the human abilities are. Very likely the organization of the intellectual

mechanisms for AI can usefully be different from that in people.

Whenever people do better than computers on some task or computers

use a lot of computation to do as well as people, this demonstrates that the

program designers lack understanding of the intellectual mechanisms required

to do the task efficiently.

Q. When did AI research start?

A. After WWII, a number of people independently started to work on

intelligent machines. The English mathematician Alan Turing may have

been the first. He gave a lecture on it in 1947. He also may have been the

first to decide that AI was best researched by programming computers rather

than by building machines. By the late 1950s, there were many researchers

on AI, and most of them were basing their work on programming computers.

Q. Does AI aim to put the human mind into the computer?

A. Some researchers say they have that objective, but maybe they are

using the phrase metaphorically. The human mind has a lot of peculiarities,

and I’m not sure anyone is serious about imitating all of them.

Q. What is the Turing test?

A. Alan Turing’s 1950 article Computing Machinery and Intelligence [Tur50]discussed conditions for considering a machine to be intelligent. He argued

that if the machine could successfully pretend to be human to a knowledge-

able observer then you certainly should consider it intelligent. This test

would satisfy most people but not all philosophers. The observer could in-

teract with the machine and a human by teletype (to avoid requiring that

the machine imitate the appearance or voice of the person), and the human

would try to persuade the observer that it was human and the machine would

try to fool the observer.

The Turing test is a one-sided test. A machine that passes the test should

certainly be considered intelligent, but a machine could still be considered

intelligent without knowing enough about humans to imitate a human.

Daniel Dennett’s book Brainchildren [Den98] has an excellent discussion

of the Turing test and the various partial Turing tests that have been im-

plemented, i.e. with restrictions on the observer’s knowledge of AI and the

subject matter of questioning. It turns out that some people are easily led

into believing that a rather dumb program is intelligent.

Q. Does AI aim at human-level intelligence?

A. Yes. The ultimate effort is to make computer programs that can solve

problems and achieve goals in the world as well as humans. However, many

people involved in particular research areas are much less ambitious.

Q. How far is AI from reaching human-level intelligence? When will it

happen?

A. A few people think that human-level intelligence can be achieved by

writing large numbers of programs of the kind people are now writing and

assembling vast knowledge bases of facts in the languages now used for ex-

pressing knowledge.

However, most AI researchers believe that new fundamental ideas are

required, and therefore it cannot be predicted when human-level intelligence

will be achieved.

Q. Are computers the right kind of machine to be made intelligent?

A. Computers can be programmed to simulate any kind of machine.

Many researchers invented non-computer machines, hoping that they

would be intelligent in different ways than the computer programs could

be. However, they usually simulate their invented machines on a computer

and come to doubt that the new machine is worth building. Because many

billions of dollars that have been spent in making computers faster and faster,

another kind of machine would have to be very fast to perform better than

a program on a computer simulating the machine.

Q. Are computers fast enough to be intelligent?

A. Some people think much faster computers are required as well as new

ideas. My own opinion is that the computers of 30 years ago were fast

enough if only we knew how to program them. Of course, quite apart from

the ambitions of AI researchers, computers will keep getting faster.

Q. What about parallel machines?

A. Machines with many processors are much faster than single proces-

sors can be. Parallelism itself presents no advantages, and parallel machines

are somewhat awkward to program. When extreme speed is required, it is

necessary to face this awkwardness.

Q. What about making a “child machine” that could improve by reading

and by learning from experience?

A. This idea has been proposed many times, starting in the 1940s. Even-

tually, it will be made to work. However, AI programs haven’t yet reached

the level of being able to learn much of what a child learns from physical

experience. Nor do present programs understand language well enough to

learn much by reading.

Q. Might an AI system be able to bootstrap itself to higher and higher

level intelligence by thinking about AI?

A. I think yes, but we aren’t yet at a level of AI at which this process can

begin.

Q. What about chess?

A. Alexander Kronrod, a Russian AI researcher, said “Chess is the Drosophilaof AI.” He was making an analogy with geneticists’ use of that fruit fly to

study inheritance. Playing chess requires certain intellectual mechanisms and

not others. Chess programs now play at grandmaster level, but they do it

with limited intellectual mechanisms compared to those used by a human

chess player, substituting large amounts of computation for understanding.

Once we understand these mechanisms better, we can build human-level

chess programs that do far less computation than do present programs.

Unfortunately, the competitive and commercial aspects of making com-

puters play chess have taken precedence over using chess as a scientific do-

main. It is as if the geneticists after 1910 had organized fruit fly races and

concentrated their efforts on breeding fruit flies that could win these races.

Q. What about Go?

A. The Chinese and Japanese game of Go is also a board game in which

the players take turns moving. Go exposes the weakness of our present under-

standing of the intellectual mechanisms involved in human game playing. Go

programs are very bad players, in spite of considerable effort (not as much as

for chess). The problem seems to be that a position in Go has to be divided

mentally into a collection of subpositions which are first analyzed separately

followed by an analysis of their interaction. Humans use this in chess also,

but chess programs consider the position as a whole. Chess programs com-

pensate for the lack of this intellectual mechanism by doing thousands or, in

the case of Deep Blue, many millions of times as much computation.

Sooner or later, AI research will overcome this scandalous weakness.

Q. Don’t some people say that AI is a bad idea?

A. The philosopher John Searle says that the idea of a non-biological ma-

chine being intelligent is incoherent. He proposes the Chinese room argument

www-formal.stanford.edu/jmc/chinese.html The philosopher Hubert Dreyfus

says that AI is impossible. The computer scientist Joseph Weizenbaum says

the idea is obscene, anti-human and immoral. Various people have said that

since artificial intelligence hasn’t reached human level by now, it must be

impossible. Still other people are disappointed that companies they invested

in went bankrupt.

Q. Aren’t computability theory and computational complexity the keys

to AI? [Note to the layman and beginners in computer science: These are

quite technical branches of mathematical logic and computer science, and

the answer to the question has to be somewhat technical.]

A. No. These theories are relevant but don’t address the fundamental

problems of AI.

In the 1930s mathematical logicians, especially Kurt G¨odel and Alan Tur-

ing, established that there did not exist algorithms that were guaranteed to

solve all problems in certain important mathematical domains. Whether a

sentence of first order logic is a theorem is one example, and whether a poly-

nomial equations in several variables has integer solutions is another. Hu-

mans solve problems in these domains all the time, and this has been offered

as an argument (usually with some decorations) that computers are intrinsi-

cally incapable of doing what people do. Roger Penrose claims this. However,

people can’t guarantee to solve arbitrary problems in these domains either.

See my Review of The Emperor’s New Mind by Roger Penrose. More essays

and reviews defending AI research are in [McC96a].

In the 1960s computer scientists, especially Steve Cook and Richard Karp

developed the theory of NP-complete problem domains. Problems in these

domains are solvable, but seem to take time exponential in the size of the

problem. Which sentences of propositional calculus are satisfiable is a basic

example of an NP-complete problem domain. Humans often solve problems

in NP-complete domains in times much shorter than is guaranteed by the

general algorithms, but can’t solve them quickly in general.

What is important for AI is to have algorithms as capable as people at

solving problems. The identification of subdomains for which good algo-

rithms exist is important, but a lot of AI problem solvers are not associated

with readily identified subdomains.

The theory of the difficulty of general classes of problems is called com-

putational complexity. So far this theory hasn’t interacted with AI as much

as might have been hoped. Success in problem solving by humans and by

AI programs seems to rely on properties of problems and problem solving

methods that the neither the complexity researchers nor the AI community

have been able to identify precisely.

Algorithmic complexity theory as developed by Solomonoff, Kolmogorov

and Chaitin (independently of one another) is also relevant. It defines the

complexity of a symbolic object as the length of the shortest program that

will generate it. Proving that a candidate program is the shortest or close

to the shortest is an unsolvable problem, but representing objects by short

programs that generate them should sometimes be illuminating even when

you can’t prove that the program is the shortest.

2 Branches of AI

Q. What are the branches of AI?

A. Here’s a list, but some branches are surely missing, because no-one

has identified them yet. Some of these may be regarded as concepts or topics

rather than full branches.

logical AI What a program knows about the world in general the facts

of the specific situation in which it must act, and its goals are all

represented by sentences of some mathematical logical language. The

program decides what to do by inferring that certain actions are ap-

propriate for achieving its goals. The first article proposing this was

[McC59]. [McC89] is a more recent summary.

[McC96b] lists some of

the concepts involved in logical aI. [Sha97] is an important text.

search AI programs often examine large numbers of possibilities, e.g. moves

in a chess game or inferences by a theorem proving program. Discover-

ies are continually made about how to do this more efficiently in various

domains.

pattern recognition When a program makes observations of some kind,

it is often programmed to compare what it sees with a pattern. For

example, a vision program may try to match a pattern of eyes and a

nose in a scene in order to find a face. More complex patterns, e.g. in

a natural language text, in a chess position, or in the history of some

event are also studied. These more complex patterns require quite

different methods than do the simple patterns that have been studied

the most.

representation Facts about the world have to be represented in some way.

Usually languages of mathematical logic are used.

inference From some facts, others can be inferred. Mathematical logical

deduction is adequate for some purposes, but new methods of non-

monotonic inference have been added to logic since the 1970s. The

simplest kind of non-monotonic reasoning is default reasoning in which

a conclusion is to be inferred by default, but the conclusion can be

withdrawn if there is evidence to the contrary. For example, when

we hear of a bird, we man infer that it can fly, but this conclusion

can be reversed when we hear that it is a penguin. It is the possibil-

ity that a conclusion may have to be withdrawn that constitutes the

non-monotonic character of the reasoning. Ordinary logical reasoning

is monotonic in that the set of conclusions that can the drawn from

a set of premises is a monotonic increasing function of the premises.

Circumscription is another form of non-monotonic reasoning.

common sense knowledge and reasoning This is the area in which AI

is farthest from human-level, in spite of the fact that it has been an

active research area since the 1950s. While there has been considerable

progress, e.g.

in developing systems of non-monotonic reasoning and

theories of action, yet more new ideas are needed. The Cyc system

contains a large but spotty collection of common sense facts.

learning from experience Programs do that. The approaches to AI based

on connectionism and neural nets specialize in that. There is also learn-

ing of laws expressed in logic.

[Mit97] is a comprehensive undergrad-

uate text on machine learning. Programs can only learn what facts

or behaviors their formalisms can represent, and unfortunately learn-

ing systems are almost all based on very limited abilities to represent

information.

planning Planning programs start with general facts about the world (es-

pecially facts about the effects of actions), facts about the particular

situation and a statement of a goal. From these, they generate a strat-

egy for achieving the goal. In the most common cases, the strategy is

just a sequence of actions.

epistemology This is a study of the kinds of knowledge that are required

for solving problems in the world.

ontology Ontology is the study of the kinds of things that exist.

In AI,

the programs and sentences deal with various kinds of objects, and

we study what these kinds are and what their basic properties are.

Emphasis on ontology begins in the 1990s.

heuristics A heuristic is a way of trying to discover something or an idea

imbedded in a program. The term is used variously in AI. Heuristic

functions are used in some approaches to search to measure how far

a node in a search tree seems to be from a goal. Heuristic predicates

that compare two nodes in a search tree to see if one is better than the

other, i.e. constitutes an advance toward the goal, may be more useful.

[My opinion].

genetic programming Genetic programming is a technique for getting pro-

grams to solve a task by mating random Lisp programs and selecting

fittest in millions of generations. It is being developed by John Koza’s

group and here’s a tutorial1.

3 Applications of AI

Q. What are the applications of AI?

A. Here are some.

game playing You can buy machines that can play master level chess for

a few hundred dollars. There is some AI in them, but they play well

against people mainly through brute force computation—looking at

hundreds of thousands of positions. To beat a world champion by

brute force and known reliable heuristics requires being able to look at

200 million positions per second.

speech recognition In the 1990s, computer speech recognition reached a

practical level for limited purposes. Thus United Airlines has replaced

its keyboard tree for flight information by a system using speech recog-

nition of flight numbers and city names. It is quite convenient. On the

the other hand, while it is possible to instruct some computers using

speech, most users have gone back to the keyboard and the mouse as

still more convenient.

1http://www.genetic-programming.com/gpanimatedtutorial.html


understanding natural language Just getting a sequence of words into a

computer is not enough. Parsing sentences is not enough either. The

computer has to be provided with an understanding of the domain

the text is about, and this is presently possible only for very limited

domains.

computer vision The world is composed of three-dimensional objects, but

the inputs to the human eye and computers’ TV cameras are two di-

mensional. Some useful programs can work solely in two dimensions,

but full computer vision requires partial three-dimensional informa-

tion that is not just a set of two-dimensional views. At present there

are only limited ways of representing three-dimensional information di-

rectly, and they are not as good as what humans evidently use.

expert systems A “knowledge engineer” interviews experts in a certain do-

main and tries to embody their knowledge in a computer program for

carrying out some task. How well this works depends on whether the

intellectual mechanisms required for the task are within the present

state of AI. When this turned out not to be so, there were many dis-

appointing results. One of the first expert systems was MYCIN in

1974, which diagnosed bacterial infections of the blood and suggested

treatments. It did better than medical students or practicing doctors,

provided its limitations were observed. Namely, its ontology included

bacteria, symptoms, and treatments and did not include patients, doc-

tors, hospitals, death, recovery, and events occurring in time. Its in-

teractions depended on a single patient being considered. Since the

experts consulted by the knowledge engineers knew about patients,

doctors, death, recovery, etc., it is clear that the knowledge engineers

forced what the experts told them into a predetermined framework. In

the present state of AI, this has to be true. The usefulness of current

expert systems depends on their users having common sense.

heuristic classification One of the most feasible kinds of expert system

given the present knowledge of AI is to put some information in one

of a fixed set of categories using several sources of information. An

example is advising whether to accept a proposed credit card purchase.

Information is available about the owner of the credit card, his record

of payment and also about the item he is buying and about the estab-

lishment from which he is buying it (e.g., about whether there have

been previous credit card frauds at this establishment).

4 More questions

Q. How is AI research done?

A. AI research has both theoretical and experimental sides. The experi-

mental side has both basic and applied aspects.

There are two main lines of research. One is biological, based on the

idea that since humans are intelligent, AI should study humans and imitate

their psychology or physiology. The other is phenomenal, based on studying

and formalizing common sense facts about the world and the problems that

the world presents to the achievement of goals. The two approaches interact

to some extent, and both should eventually succeed. It is a race, but both

racers seem to be walking.

Q. What are the relations between AI and philosophy?

A. AI has many relations with philosophy, especially modern analytic

philosophy. Both study mind, and both study common sense. The best

reference is [Tho03].

Q. How are AI and logic programming related?

A. At the very least, logic programming provides useful programming

languages (mainly Prolog).

Beyond that, sometimes a theory T useful in AI can be expressed as a col-

lection H of Horn clauses, and goal G to be achieved can be expressed as that

of finding values of variables x1 . . . xn satisfying an expression g(x1 . . . xn).

The problem can sometimes be solved by running the Prolog program con-

sisting of G and H.

There are two possible obstacles to regarding AI as logic programming.

First, Horn theories do not exhaust first order logic. Second, the Prolog

program expressing the theory may be extremely inefficient. More elaborate

control than just executing the program that expresses the theory is often

needed. Map coloring provides examples.

Q. What should I study before or while learning AI?

A. Study mathematics, especially mathematical logic. The more you

learn about sciences, e.g. physics or biology, the better. For the biological

approaches to AI, study psychology and the physiology of the nervous system.

Learn some programming languages—at least C, Lisp and Prolog. It is also a

good idea to learn one basic machine language. Jobs are likely to depend on

knowing the languages currently in fashion. In the late 1990s, these include

C++ and Java.

Q. What is a good textbook on AI?

A. Artificial Intelligence by Stuart Russell and Peter Norvig, Prentice Hall

is the most commonly used textbbook in 1997. The general views expressed

there do not exactly correspond to those of this essay. Artificial Intelligence:

A New Synthesis by Nils Nilsson, Morgan Kaufman, may be easier to read.

Some people prefer Computational Intelligence by David Poole, Alan Mack-

worth and Randy Goebel, Oxford, 1998.

Q. What organizations and publications are concerned with AI?

A. The American Association for Artificial Intelligence (AAAI)2, the Eu-

ropean Coordinating Committee for Artificial Intelligence (ECCAI)3 and the

Society for Artificial Intelligence and Simulation of Behavior (AISB)4 are

scientific societies concerned with AI research. The Association for Comput-

ing Machinery (ACM) has a special interest group on artificial intelligence

SIGART5.

The International Joint Conference on AI (IJCAI)6 is the main inter-

national conference. The AAAI7 runs a US National Conference on AI.

Electronic Transactions on Artificial Intelligence 8, Artificial Intelligence 9,

and Journal of Artificial Intelligence Research 10, and IEEE Transactions on

Pattern Analysis and Machine Intelligence11 are four of the main journals

publishing AI research papers. I have not yet found everything that should

be in this paragraph.

Page of Positive Reviews 12 lists papers that experts have found impor-

tant.

Funding a Revolution: Government Support for Computing Research by a

committee of the National Research covers support for AI research in Chapter

2http://www.aaai.org

3http://www.eccai.org/

4http://www.cogs.susx.ac.uk/aisb

5http://www.acm.org/sigart

6http://www.ijcai.org

7http://www.aaai.org

8http://www.ida.liu.se/ext/etai/

9http://www.elsevier.nl/locate/artint/

10http://www.jair.org/

11http://computer.org/tpami/

12http://www.cs.utexas.edu/users/vl/ppr/


9.13

References

[Den98] Daniel Dennett. Brainchildren: Essays on Designing Minds. MIT

Press, 1998.

[Jen98]

Arthur R. Jensen. Does IQ matter? Commentary, pages 20–21,

November 1998. The reference is just to Jensen’s comment—one

of many.

[McC59]

John McCarthy. Programs with Common Sense14. In Mechani-

sation of Thought Processes, Proceedings of the Symposium of the

National Physics Laboratory, pages 77–84, London, U.K., 1959.

Her Majesty’s Stationery Office. Reprinted in [McC90].

[McC89]

John McCarthy. Artificial Intelligence, Logic and Formalizing

Common Sense15. In Richmond Thomason, editor, Philosophical

Logic and Artificial Intelligence. Kl¨uver Academic, 1989.

[McC90]

John McCarthy. Formalizing Common Sense: Papers by John

McCarthy. Ablex Publishing Corporation, 1990.

[McC96a] John McCarthy. Defending AI research : a collection of essays

and reviews. CSLI lecture notes: no. 49. Center for the Study

of Language and Information, 1996. distributed by Cambridge

University Press.

[McC96b] John McCarthy. Concepts of Logical AI16, 1996. Web only for

now but may be referenced.

[Mit97]

Tom Mitchell. Machine Learning. McGraw-Hill, 1997.

[Sha97] Murray Shanahan. Solving the Frame Problem, a mathematical

investigation of the common sense law of inertia. M.I.T. Press,

1997.

13http://www.nap.edu/readingroom/books/far/ch9.html

14http://www-formal.stanford.edu/jmc/mcc59.html

15http://www-formal.stanford.edu/jmc/ailogic.html

16http://www-formal.stanford.edu/jmc/concepts-ai.html


[Tho03] Richmond Thomason. Logic and artificial intelligence.

In Ed-

ward N. Zalta, editor, The Stanford Encyclopedia of Philosophy.

2003. http://plato.stanford.edu/entries/logic-ai/.

[Tur50]

Alan Turing. Computing machinery and intelligence. Mind, 1950.

/@steam.stanford.edu:/u/ftp/jmc/whatisai.tex: begun Sat Nov 23 10:30:17 1996, latexed November 12, 2007 at 2:05 a.m.