Physical symbol system
Encyclopedia
A physical symbol system (also called a formal system
) takes physical patterns (symbols), combining them into structures (expressions) and manipulating them (using processes) to produce new expressions.
The physical symbol system hypothesis (PSSH) is a position in the philosophy of artificial intelligence
formulated by Allen Newell
and Herbert Simon
. They wrote:
This claim implies both that human thinking is a kind of symbol manipulation (because a symbol system is necessary for intelligence) and that machines can be intelligent (because a symbol system is sufficient for intelligence).
The idea has philosophical roots in Hobbes (who claimed reasoning was "nothing more than reckoning"), Leibniz (who attempted to create a logical calculus of all human ideas), Hume
(who thought perception could be reduced to "atomic impressions") and even Kant
(who analyzed all experience as controlled by formal rules). The latest version is called the computational theory of mind
, associated with philosophers Hilary Putnam
and Jerry Fodor
.
The hypothesis has been criticized strongly by various parties, but is a core part of AI research. A common critical view is that the hypothesis seems appropriate for higher-level intelligence such as playing chess, but less appropriate for commonplace intelligence such as vision. A distinction is usually made between the kind of high level symbols that directly correspond with objects in the world, such as
.
The physical symbol system hypothesis claims that both of these are also examples of physical symbol systems:
and Herbert Simon
that "symbol manipulation" was the essence of both human and machine intelligence: the development of artificial intelligence
programs and psychological experiments on human beings.
First, in the early decades of AI research there were a number of very successful programs that used high level symbol processing, such as Newell
and Herbert Simon
's General Problem Solver
or Terry Winograd
's SHRDLU
. John Haugeland
named this kind of AI research "Good Old Fashioned AI" or GOFAI
. Expert system
s and logic programming
are descendants of this tradition. The success of these programs suggested that symbol processing systems could simulate any intelligent action.
And second, psychological
experiments carried out at the same time found that, for difficult problems in logic, planning or any kind of "puzzle solving", people used this kind of symbol processing as well. AI researchers were able to simulate the step by step problem solving skills of people with computer programs. This collaboration and the issues it raised eventually would lead to the creation of the field of cognitive science
. (This type of research was called "cognitive simulation.") This line of research suggested that human problem solving consisted primarily of the manipulation of high level symbols.
or denotation
and can be composed with other symbols to create more complex symbols.
However, it is also possible to interpret the hypothesis as referring to the simple abstract 0s and 1s in the memory of a digital computer or the stream of 0s and 1s passing through the perceptual apparatus of a robot. These are, in some sense, symbols as well, although it is not always possible to determine exactly what the symbols are standing for. In this version of the hypothesis, no distinction is being made between "symbols" and "signals", as David Touretzky and Dean Pomerleau explain.
Under this interpretation, the physical symbol system hypothesis asserts merely that intelligence can be digitized. This is a weaker claim. Indeed, Touretzky and Pomerleau write that if symbols and signals are the same thing, then "[s]ufficiency is a given, unless one is a dualist or some other sort of mystic, because physical symbol systems are Turing-universal
." The widely accepted Church–Turing thesis holds that any Turing-universal
system can simulate any conceivable process that can be digitized, given enough time and memory. Since any digital computer is Turing-universal
, any digital computer can, in theory, simulate anything that can be digitized to a sufficient level of precision, including the behavior of intelligent organisms. The necessary condition of the physical symbol systems hypothesis can likewise be finessed, since we are willing to accept almost any signal as a form of "symbol" and all intelligent biological systems have signal pathways.
attacked the necessary condition of the physical symbol system hypothesis, calling it "the psychological assumption" and defining it thus:
Dreyfus refuted this by showing that human intelligence and expertise depended primarily on unconscious instincts rather than conscious symbolic manipulation. Experts solve problems quickly by using their intuitions, rather than step-by-step trial and error searches. Dreyfus argued that these unconscious skills would never be captured in formal rules.
's Chinese Room
argument, presented in 1980, attempted to show that a program (or any physical symbol system) could not be said to "understand" the symbols that it uses; that the symbols have no meaning for the machine, and so the machine can never be truly intelligent.
s that used symbols to represent the world and plan actions (such as the Stanford Cart). These projects had limited success. In the middle eighties, Rodney Brooks
of MIT was able to build robots that had superior ability to move and survive without the use of symbolic reasoning at all. Brooks (and others, such as Hans Moravec
) discovered that our most basic skills of motion, survival, perception, balance and so on did not seem to require high level symbols at all, that in fact, the use of high level symbols was more complicated and less successful.
In a 1990 paper Elephants Don't Play Chess, robotics researcher Rodney Brooks
took direct aim at the physical symbol system hypothesis, arguing that symbols are not always necessary since "the world is its own best model. It is always exactly up to date. It always has every detail there is to be known. The trick is to sense it appropriately and often enough."
, Mark Turner
and others have argued that our abstract skills in areas such as mathematics, ethics and philosophy
depend on unconscious skills that derive from the body, and that conscious symbol manipulation is only a small part of our intelligence.
Formal system
In formal logic, a formal system consists of a formal language and a set of inference rules, used to derive an expression from one or more other premises that are antecedently supposed or derived . The axioms and rules may be called a deductive apparatus...
) takes physical patterns (symbols), combining them into structures (expressions) and manipulating them (using processes) to produce new expressions.
The physical symbol system hypothesis (PSSH) is a position in the philosophy of artificial intelligence
Philosophy of artificial intelligence
The philosophy of artificial intelligence attempts to answer such questions as:* Can a machine act intelligently? Can it solve any problem that a person would solve by thinking?...
formulated by Allen Newell
Allen Newell
Allen Newell was a researcher in computer science and cognitive psychology at the RAND corporation and at Carnegie Mellon University’s School of Computer Science, Tepper School of Business, and Department of Psychology...
and Herbert Simon
Herbert Simon
Herbert Alexander Simon was an American political scientist, economist, sociologist, and psychologist, and professor—most notably at Carnegie Mellon University—whose research ranged across the fields of cognitive psychology, cognitive science, computer science, public administration, economics,...
. They wrote:
This claim implies both that human thinking is a kind of symbol manipulation (because a symbol system is necessary for intelligence) and that machines can be intelligent (because a symbol system is sufficient for intelligence).
The idea has philosophical roots in Hobbes (who claimed reasoning was "nothing more than reckoning"), Leibniz (who attempted to create a logical calculus of all human ideas), Hume
David Hume
David Hume was a Scottish philosopher, historian, economist, and essayist, known especially for his philosophical empiricism and skepticism. He was one of the most important figures in the history of Western philosophy and the Scottish Enlightenment...
(who thought perception could be reduced to "atomic impressions") and even Kant
KANT
KANT is a computer algebra system for mathematicians interested in algebraic number theory, performing sophisticated computations in algebraic number fields, in global function fields, and in local fields. KASH is the associated command line interface...
(who analyzed all experience as controlled by formal rules). The latest version is called the computational theory of mind
Computational theory of mind
In philosophy, the computational theory of mind is the view that the human mind is an information processing system and that thinking is a form of computing. The theory was proposed in its modern form by Hilary Putnam in 1961 and developed by Jerry Fodor in the 60s and 70s...
, associated with philosophers Hilary Putnam
Hilary Putnam
Hilary Whitehall Putnam is an American philosopher, mathematician and computer scientist, who has been a central figure in analytic philosophy since the 1960s, especially in philosophy of mind, philosophy of language, philosophy of mathematics, and philosophy of science...
and Jerry Fodor
Jerry Fodor
Jerry Alan Fodor is an American philosopher and cognitive scientist. He holds the position of State of New Jersey Professor of Philosophy at Rutgers University and is the author of many works in the fields of philosophy of mind and cognitive science, in which he has laid the groundwork for the...
.
The hypothesis has been criticized strongly by various parties, but is a core part of AI research. A common critical view is that the hypothesis seems appropriate for higher-level intelligence such as playing chess, but less appropriate for commonplace intelligence such as vision. A distinction is usually made between the kind of high level symbols that directly correspond with objects in the world, such as
Neural network
The term neural network was traditionally used to refer to a network or circuit of biological neurons. The modern usage of the term often refers to artificial neural networks, which are composed of artificial neurons or nodes...
.
Examples of physical symbol systems
Examples of physical symbol systems include:- Formal logicFormal logicClassical or traditional system of determining the validity or invalidity of a conclusion deduced from two or more statements...
: the symbols are words like "and", "or", "not", "for all x" and so on. The expressions are statements in formal logic which can be true or false. The processes are the rules of logical deduction. - AlgebraElementary algebraElementary algebra is a fundamental and relatively basic form of algebra taught to students who are presumed to have little or no formal knowledge of mathematics beyond arithmetic. It is typically taught in secondary school under the term algebra. The major difference between algebra and...
: the symbols are "+", "×", "x", "y", "1", "2", "3", etc. The expressions are equations. The processes are the rules of algebra, that allow one to manipulate a mathematical expression and retain its truth. - A digital computer: the symbols are zeros and ones of computer memory, the processes are the operations of the CPU that change memory.
- ChessChessChess is a two-player board game played on a chessboard, a square-checkered board with 64 squares arranged in an eight-by-eight grid. It is one of the world's most popular games, played by millions of people worldwide at home, in clubs, online, by correspondence, and in tournaments.Each player...
: the symbols are the pieces, the processes are the legal chess moves, the expressions are the positions of all the pieces on the board.
The physical symbol system hypothesis claims that both of these are also examples of physical symbol systems:
- Intelligent human thought: the symbols are encoded in our brains. The expressions are thoughtThought"Thought" generally refers to any mental or intellectual activity involving an individual's subjective consciousness. It can refer either to the act of thinking or the resulting ideas or arrangements of ideas. Similar concepts include cognition, sentience, consciousness, and imagination...
s. The processes are the mental operations of thinking. - A running artificial intelligenceArtificial intelligenceArtificial intelligence is the intelligence of machines and the branch of computer science that aims to create it. AI textbooks define the field as "the study and design of intelligent agents" where an intelligent agent is a system that perceives its environment and takes actions that maximize its...
program: The symbols are data. The expressions are more data. The processes are programs that manipulate the data.
Newell and Simon
Two lines of evidence suggested to Allen NewellAllen Newell
Allen Newell was a researcher in computer science and cognitive psychology at the RAND corporation and at Carnegie Mellon University’s School of Computer Science, Tepper School of Business, and Department of Psychology...
and Herbert Simon
Herbert Simon
Herbert Alexander Simon was an American political scientist, economist, sociologist, and psychologist, and professor—most notably at Carnegie Mellon University—whose research ranged across the fields of cognitive psychology, cognitive science, computer science, public administration, economics,...
that "symbol manipulation" was the essence of both human and machine intelligence: the development of artificial intelligence
Artificial intelligence
Artificial intelligence is the intelligence of machines and the branch of computer science that aims to create it. AI textbooks define the field as "the study and design of intelligent agents" where an intelligent agent is a system that perceives its environment and takes actions that maximize its...
programs and psychological experiments on human beings.
First, in the early decades of AI research there were a number of very successful programs that used high level symbol processing, such as Newell
Allen Newell
Allen Newell was a researcher in computer science and cognitive psychology at the RAND corporation and at Carnegie Mellon University’s School of Computer Science, Tepper School of Business, and Department of Psychology...
and Herbert Simon
Herbert Simon
Herbert Alexander Simon was an American political scientist, economist, sociologist, and psychologist, and professor—most notably at Carnegie Mellon University—whose research ranged across the fields of cognitive psychology, cognitive science, computer science, public administration, economics,...
's General Problem Solver
General Problem Solver
General Problem Solver was a computer program created in 1959 by Herbert Simon, J.C. Shaw, and Allen Newell intended to work as a universal problem solver machine. Any formalized symbolic problem can be solved, in principle, by GPS. For instance: theorems proof, geometric problems and chess...
or Terry Winograd
Terry Winograd
Terry Allen Winograd is an American professor of computer science at Stanford University, and co-director of the Stanford Human-Computer Interaction Group...
's SHRDLU
SHRDLU
SHRDLU was an early natural language understanding computer program, developed by Terry Winograd at MIT from 1968-1970. In it, the user carries on a conversation with the computer, moving objects, naming collections and querying the state of a simplified "blocks world", essentially a virtual box...
. John Haugeland
John Haugeland
John Haugeland was a professor of philosophy at the University of Chicago from 1999 until his death. He was chair of the philosophy department from 2004-2007. He spent at most of his career teaching at the University of Pittsburgh...
named this kind of AI research "Good Old Fashioned AI" or GOFAI
GOFAI
In artificial intelligence research, GOFAI describes the oldest original approach to achieving artificial intelligence, based on logic and problem solving...
. Expert system
Expert system
In artificial intelligence, an expert system is a computer system that emulates the decision-making ability of a human expert. Expert systems are designed to solve complex problems by reasoning about knowledge, like an expert, and not by following the procedure of a developer as is the case in...
s and logic programming
Logic programming
Logic programming is, in its broadest sense, the use of mathematical logic for computer programming. In this view of logic programming, which can be traced at least as far back as John McCarthy's [1958] advice-taker proposal, logic is used as a purely declarative representation language, and a...
are descendants of this tradition. The success of these programs suggested that symbol processing systems could simulate any intelligent action.
And second, psychological
Psychology
Psychology is the study of the mind and behavior. Its immediate goal is to understand individuals and groups by both establishing general principles and researching specific cases. For many, the ultimate goal of psychology is to benefit society...
experiments carried out at the same time found that, for difficult problems in logic, planning or any kind of "puzzle solving", people used this kind of symbol processing as well. AI researchers were able to simulate the step by step problem solving skills of people with computer programs. This collaboration and the issues it raised eventually would lead to the creation of the field of cognitive science
Cognitive science
Cognitive science is the interdisciplinary scientific study of mind and its processes. It examines what cognition is, what it does and how it works. It includes research on how information is processed , represented, and transformed in behaviour, nervous system or machine...
. (This type of research was called "cognitive simulation.") This line of research suggested that human problem solving consisted primarily of the manipulation of high level symbols.
Turing completeness
In Newell and Simon's arguments, the "symbols" that the hypothesis is referring to are physical objects that represent things in the world, symbols such asMeaning (semiotics)
In semiotics, the meaning of a sign is its place in a sign relation, in other words, the set of roles that it occupies within a given sign relation. This statement holds whether sign is taken to mean a sign type or a sign token...
or denotation
Denotation
This word has distinct meanings in other fields: see denotation . For the opposite of Denotation see Connotation.*In logic, linguistics and semiotics, the denotation of a word or phrase is a part of its meaning; however, the part referred to varies by context:** In grammar and literary theory, the...
and can be composed with other symbols to create more complex symbols.
However, it is also possible to interpret the hypothesis as referring to the simple abstract 0s and 1s in the memory of a digital computer or the stream of 0s and 1s passing through the perceptual apparatus of a robot. These are, in some sense, symbols as well, although it is not always possible to determine exactly what the symbols are standing for. In this version of the hypothesis, no distinction is being made between "symbols" and "signals", as David Touretzky and Dean Pomerleau explain.
Under this interpretation, the physical symbol system hypothesis asserts merely that intelligence can be digitized. This is a weaker claim. Indeed, Touretzky and Pomerleau write that if symbols and signals are the same thing, then "[s]ufficiency is a given, unless one is a dualist or some other sort of mystic, because physical symbol systems are Turing-universal
Turing completeness
In computability theory, a system of data-manipulation rules is said to be Turing complete or computationally universal if and only if it can be used to simulate any single-taped Turing machine and thus in principle any computer. A classic example is the lambda calculus...
." The widely accepted Church–Turing thesis holds that any Turing-universal
Turing completeness
In computability theory, a system of data-manipulation rules is said to be Turing complete or computationally universal if and only if it can be used to simulate any single-taped Turing machine and thus in principle any computer. A classic example is the lambda calculus...
system can simulate any conceivable process that can be digitized, given enough time and memory. Since any digital computer is Turing-universal
Turing completeness
In computability theory, a system of data-manipulation rules is said to be Turing complete or computationally universal if and only if it can be used to simulate any single-taped Turing machine and thus in principle any computer. A classic example is the lambda calculus...
, any digital computer can, in theory, simulate anything that can be digitized to a sufficient level of precision, including the behavior of intelligent organisms. The necessary condition of the physical symbol systems hypothesis can likewise be finessed, since we are willing to accept almost any signal as a form of "symbol" and all intelligent biological systems have signal pathways.
Criticism
Nils Nilsson has identified four main "themes" or grounds in which the physical symbol system hypothesis has been attacked.- The "erroneous claim that the [physical symbol system hypothesis] lacks symbol groundingSymbol groundingThe Symbol Grounding Problem is related to the problem of how words get their meanings, and hence to the problem of what meaning itself really is. The problem of meaning is in turn related to the problem of consciousness, or how it is that mental states are meaningful...
" which is presumed to be a requirement for general intelligent action. - The common belief that AI requires non-symbolic processing (that which can be supplied by a connectionist architecture for instance).
- The common statement that the brain is simply not a computer and that "computation as it is currently understood, does not provide an appropriate model for intelligence".
- And last of all that it is also believed in by some that the brain is essentially mindless, most of what takes place are chemical reactions and that human intelligent behaviour is analogous to the intelligent behaviour displayed for example by ant colonies.
Dreyfus and the primacy of unconscious skills
Hubert DreyfusHubert Dreyfus
Hubert Lederer Dreyfus is an American philosopher. He is a professor of philosophy at the University of California, Berkeley....
attacked the necessary condition of the physical symbol system hypothesis, calling it "the psychological assumption" and defining it thus:
- The mind can be viewed as a device operating on bits of information according to formal rules.
Dreyfus refuted this by showing that human intelligence and expertise depended primarily on unconscious instincts rather than conscious symbolic manipulation. Experts solve problems quickly by using their intuitions, rather than step-by-step trial and error searches. Dreyfus argued that these unconscious skills would never be captured in formal rules.
Searle and his Chinese Room
John SearleJohn Searle
John Rogers Searle is an American philosopher and currently the Slusser Professor of Philosophy at the University of California, Berkeley.-Biography:...
's Chinese Room
Chinese room
The Chinese room is a thought experiment by John Searle, which first appeared in his paper "Minds, Brains, and Programs", published in Behavioral and Brain Sciences in 1980...
argument, presented in 1980, attempted to show that a program (or any physical symbol system) could not be said to "understand" the symbols that it uses; that the symbols have no meaning for the machine, and so the machine can never be truly intelligent.
Brooks and the roboticists
In the sixties and seventies, several laboratories attempted to build robotRobot
A robot is a mechanical or virtual intelligent agent that can perform tasks automatically or with guidance, typically by remote control. In practice a robot is usually an electro-mechanical machine that is guided by computer and electronic programming. Robots can be autonomous, semi-autonomous or...
s that used symbols to represent the world and plan actions (such as the Stanford Cart). These projects had limited success. In the middle eighties, Rodney Brooks
Rodney Brooks
Rodney Allen Brooks is the former Panasonic professor of robotics at the Massachusetts Institute of Technology. Since 1986 he has authored a series of highly influential papers which have inaugurated a fundamental shift in artificial intelligence research...
of MIT was able to build robots that had superior ability to move and survive without the use of symbolic reasoning at all. Brooks (and others, such as Hans Moravec
Hans Moravec
Hans Moravec is an adjunct faculty member at the Robotics Institute of Carnegie Mellon University. He is known for his work on robotics, artificial intelligence, and writings on the impact of technology. Moravec also is a futurist with many of his publications and predictions focusing on...
) discovered that our most basic skills of motion, survival, perception, balance and so on did not seem to require high level symbols at all, that in fact, the use of high level symbols was more complicated and less successful.
In a 1990 paper Elephants Don't Play Chess, robotics researcher Rodney Brooks
Rodney Brooks
Rodney Allen Brooks is the former Panasonic professor of robotics at the Massachusetts Institute of Technology. Since 1986 he has authored a series of highly influential papers which have inaugurated a fundamental shift in artificial intelligence research...
took direct aim at the physical symbol system hypothesis, arguing that symbols are not always necessary since "the world is its own best model. It is always exactly up to date. It always has every detail there is to be known. The trick is to sense it appropriately and often enough."
Embodied philosophy
George LakoffGeorge Lakoff
George P. Lakoff is an American cognitive linguist and professor of linguistics at the University of California, Berkeley, where he has taught since 1972...
, Mark Turner
Mark Turner
Mark Turner may refer to:*Mark Turner *Mark Turner *Mark Turner *Mark Turner *Mark Turner , jazz saxophonist...
and others have argued that our abstract skills in areas such as mathematics, ethics and philosophy
Philosophy
Philosophy is the study of general and fundamental problems, such as those connected with existence, knowledge, values, reason, mind, and language. Philosophy is distinguished from other ways of addressing such problems by its critical, generally systematic approach and its reliance on rational...
depend on unconscious skills that derive from the body, and that conscious symbol manipulation is only a small part of our intelligence.