A physical symbol system (also called a
formal system
A formal system is an abstract structure used for inferring theorems from axioms according to a set of rules. These rules, which are used for carrying out the inference of theorems from axioms, are the logical calculus of the formal system.
A form ...
) 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
The philosophy of artificial intelligence is a branch of the philosophy of technology that explores artificial intelligence and its implications for knowledge and understanding of intelligence, ethics, consciousness, epistemology, and free will. ...
formulated by
Allen Newell
Allen Newell (March 19, 1927 – July 19, 1992) 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 ...
and
Herbert A. Simon
Herbert Alexander Simon (June 15, 1916 – February 9, 2001) was an American political scientist, with a Ph.D. in political science, whose work also influenced the fields of computer science, economics, and cognitive psychology. His primary ...
. 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
In logic and mathematics, necessity and sufficiency are terms used to describe a conditional or implicational relationship between two statements. For example, in the conditional statement: "If then ", is necessary for , because the truth of ...
for intelligence).
The idea has philosophical roots in
Hobbes
Thomas Hobbes ( ; 5/15 April 1588 – 4/14 December 1679) was an English philosopher, considered to be one of the founders of modern political philosophy. Hobbes is best known for his 1651 book ''Leviathan'', in which he expounds an influe ...
(who claimed reasoning was "nothing more than reckoning"),
Leibniz
Gottfried Wilhelm (von) Leibniz . ( – 14 November 1716) was a German polymath active as a mathematician, philosopher, scientist and diplomat. He is one of the most prominent figures in both the history of philosophy and the history of mathema ...
(who attempted to create a logical calculus of all human ideas),
Hume (who thought perception could be reduced to "atomic impressions") and even
Kant
Immanuel Kant (, , ; 22 April 1724 – 12 February 1804) was a German Philosophy, philosopher and one of the central Age of Enlightenment, Enlightenment thinkers. Born in Königsberg, Kant's comprehensive and systematic works in epistemolo ...
(who analyzed all experience as controlled by formal rules). The latest version is called the
computational theory of mind
In philosophy of mind, the computational theory of mind (CTM), also known as computationalism, is a family of views that hold that the human mind is an information processing system and that cognition and consciousness together are a form of comp ...
, associated with philosophers
Hilary Putnam
Hilary Whitehall Putnam (; July 31, 1926 – March 13, 2016) was an American philosopher, mathematician, and computer scientist, and a major figure in analytic philosophy in the second half of the 20th century. He made significant contributions ...
and
Jerry Fodor
Jerry Alan Fodor (; April 22, 1935 – November 29, 2017) was an American philosopher and the author of many crucial works in the fields of philosophy of mind and cognitive science. His writings in these fields laid the groundwork for the modu ...
.
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
and
and the more complex "symbols" that are present in a machine like a
neural network
A neural network is a network or circuit of biological neurons, or, in a modern sense, an artificial neural network, composed of artificial neurons or nodes. Thus, a neural network is either a biological neural network, made up of biological ...
.
Examples
Examples of physical symbol systems include:
*
Formal logic
Logic is the study of correct reasoning. It includes both formal and informal logic. Formal logic is the science of deductively valid inferences or of logical truths. It is a formal science investigating how conclusions follow from premis ...
: 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.
*
Algebra
Algebra () is one of the broad areas of mathematics. Roughly speaking, algebra is the study of mathematical symbols and the rules for manipulating these symbols in formulas; it is a unifying thread of almost all of mathematics.
Elementary a ...
: 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
A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations ( computation) automatically. Modern digital electronic computers can perform generic sets of operations known as programs. These pro ...
: the symbols are zeros and ones of computer memory, the processes are the operations of the
CPU that change memory.
*
Chess
Chess is a board game for two players, called White and Black, each controlling an army of chess pieces in their color, with the objective to checkmate the opponent's king. It is sometimes called international chess or Western chess to disti ...
: 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
thought
In their most common sense, the terms thought and thinking refer to conscious cognitive processes that can happen independently of sensory stimulation. Their most paradigmatic forms are judging, reasoning, concept formation, problem solving, a ...
s. The processes are the mental operations of thinking.
* A running
artificial intelligence
Artificial intelligence (AI) is intelligence—perceiving, synthesizing, and inferring information—demonstrated by machines, as opposed to intelligence displayed by animals and humans. Example tasks in which this is done include speech re ...
program: the symbols are data. The expressions are more data. The processes are programs that manipulate the data.
Arguments in favor of the physical symbol system hypothesis
Newell and Simon
Two lines of evidence suggested to
Allen Newell
Allen Newell (March 19, 1927 – July 19, 1992) 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 ...
and
Herbert A. Simon
Herbert Alexander Simon (June 15, 1916 – February 9, 2001) was an American political scientist, with a Ph.D. in political science, whose work also influenced the fields of computer science, economics, and cognitive psychology. His primary ...
that "symbol manipulation" was the essence of both human and machine intelligence: the development of
artificial intelligence
Artificial intelligence (AI) is intelligence—perceiving, synthesizing, and inferring information—demonstrated by machines, as opposed to intelligence displayed by animals and humans. Example tasks in which this is done include speech re ...
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 A. Simon
Herbert Alexander Simon (June 15, 1916 – February 9, 2001) was an American political scientist, with a Ph.D. in political science, whose work also influenced the fields of computer science, economics, and cognitive psychology. His primary ...
's
General Problem Solver General Problem Solver (GPS) is a computer program created in 1959 by Herbert A. Simon, J. C. Shaw, and Allen Newell (RAND Corporation) intended to work as a universal problem solver machine. In contrast to the former Logic Theorist project, the GP ...
or
Terry Winograd
Terry Allen Winograd (born February 24, 1946) is an American professor of computer science at Stanford University, and co-director of the Stanford Human–Computer Interaction Group. He is known within the philosophy of mind and artificial intell ...
's
SHRDLU
SHRDLU was an early natural-language understanding computer program, developed by Terry Winograd at MIT in 1968–1970. In the program, the user carries on a conversation with the computer, moving objects, naming collections and querying the ...
.
John Haugeland
John Haugeland (; March 13, 1945 – June 23, 2010) was a professor of philosophy, specializing in the philosophy of mind, cognitive science, phenomenology, and Heidegger. He spent most of his career at the University of Pittsburgh, followed by ...
named this kind of AI research "Good Old Fashioned AI" or
GOFAI
GOFAI is an acronym for "Good Old-Fashioned Artificial Intelligence" invented by the philosopher John Haugeland in his 1985 book, ''Artificial Intelligence: The Very Idea''. Technically, GOFAI refers only to a restricted kind of symbolic AI, name ...
.
Expert system
In artificial intelligence, an expert system is a computer system emulating the decision-making ability of a human expert.
Expert systems are designed to solve complex problems by reasoning through bodies of knowledge, represented mainly as if†...
s and
logic programming
Logic programming is a programming paradigm which is largely based on formal logic. Any program written in a logic programming language is a set of sentences in logical form, expressing facts and rules about some problem domain. Major logic prog ...
are descendants of this tradition. The success of these programs suggested that symbol processing systems could simulate any intelligent action.
And second,
psychological
Psychology is the scientific study of mind and behavior. Psychology includes the study of conscious and unconscious phenomena, including feelings and thoughts. It is an academic discipline of immense scope, crossing the boundaries between t ...
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
Artificial intelligence (AI) is intelligence—perceiving, synthesizing, and inferring information—demonstrated by machines, as opposed to intelligence displayed by animals and humans. Example tasks in which this is done include speech rec ...
".) This line of research suggested that human problem solving consisted primarily of the manipulation of high level symbols.
Symbols vs. signals
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 as
that have a recognizable
meaning or
denotation
In linguistics and philosophy, the denotation of an expression is its literal meaning. For instance, the English word "warm" denotes the property of being warm. Denotation is contrasted with other aspects of meaning including connotation. For inst ...
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
Dean may refer to:
People
* Dean (given name)
* Dean (surname), a surname of Anglo-Saxon English origin
* Dean (South Korean singer), a stage name for singer Kwon Hyuk
* Dean Delannoit, a Belgian singer most known by the mononym Dean
Titles
* ...
explain.
[
Reconstructing Physical Symbol Systems
David S. Touretzky and Dean A. Pomerleau
Computer Science Department
Carnegie Mellon University
Cognitive Science 18(2):345–353, 1994.
https://www.cs.cmu.edu/~dst/pubs/simon-reply-www.ps.gz
]
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 "
fficiency 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.
]
Criticism
The physical symbol system hypothesis applies only to systems with high level symbols, and not to signal processing systems (as found in most robots) or neural networks and other statistical machine learning methods. The criticisms below are only directed against symbolic AI
In artificial intelligence, symbolic artificial intelligence is the term for the collection of all methods in artificial intelligence research that are based on high-level symbolic (human-readable) representations of problems, logic and search. S ...
, and not at these more modern forms of AI.
Nils Nilsson has identified four main "themes" or grounds in which the physical symbol system hypothesis has been attacked.
#The "erroneous claim that the hysical symbol system hypothesislacks symbol grounding" which is presumed to be a requirement for general intelligent action. (Such as Brooks, below)
#The common belief that AI requires non-symbolic processing (that which can be supplied by a connectionist architecture for instance). (Such as Dreyfus, below)
#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 Dreyfus
Hubert Lederer Dreyfus (; October 15, 1929 – April 22, 2017) was an American philosopher and professor of philosophy at the University of California, Berkeley. His main interests included phenomenology, existentialism and the philosophy of bo ...
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.[, , . See also , and ]
However, advances in sentient and common sense reasoning
In artificial intelligence (AI), commonsense reasoning is a human-like ability to make presumptions about the type and essence of ordinary situations humans encounter every day. These assumptions include judgments about the nature of physical objec ...
has set forth empirical data that scholars are seriously considering in juxtaposition to "the psychological assumption".
Searle and his Chinese room
John Searle
John Rogers Searle (; born July 31, 1932) is an American philosopher widely noted for contributions to the philosophy of language, philosophy of mind, and social philosophy. He began teaching at UC Berkeley in 1959, and was Willis S. and Mario ...
'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 themselves have no meaning or semantic content, and so the machine can never be truly intelligent from symbol manipulation alone.
Brooks and the roboticists
In the sixties and seventies, several laboratories attempted to build robot
A robot is a machine—especially one programmable by a computer—capable of carrying out a complex series of actions automatically. A robot can be guided by an external control device, or the control may be embedded within. Robots may be c ...
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 Allen Brooks (born 30 December 1954) is an Australian roboticist, Fellow of the Australian Academy of Science, author, and robotics entrepreneur, most known for popularizing the actionist approach to robotics. He was a Panasonic Profes ...
of MIT
The Massachusetts Institute of Technology (MIT) is a private land-grant research university in Cambridge, Massachusetts. Established in 1861, MIT has played a key role in the development of modern technology and science, and is one of the m ...
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 Peter Moravec (born November 30, 1948, Kautzen, Austria) is an adjunct faculty member at the Robotics Institute of Carnegie Mellon University in Pittsburgh, USA. He is known for his work on robotics, artificial intelligence, and writings 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 pape
Elephants Don't Play Chess
robotics researcher Rodney Brooks
Rodney Allen Brooks (born 30 December 1954) is an Australian roboticist, Fellow of the Australian Academy of Science, author, and robotics entrepreneur, most known for popularizing the actionist approach to robotics. He was a Panasonic Profes ...
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."
Connectionism
Embodied philosophy
George Lakoff
George Philip Lakoff (; born May 24, 1941) is an American cognitive linguistics, cognitive linguist and philosopher, best known for his thesis that people's lives are significantly influenced by the conceptual metaphors they use to explain comple ...
, Mark Turner and others have argued that our abstract skills in areas such as mathematic
Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in modern mathematics ...
s, ethic
Ethics or moral philosophy is a branch of philosophy that "involves systematizing, defending, and recommending concepts of right and wrong behavior".''Internet Encyclopedia of Philosophy'' The field of ethics, along with aesthetics, concerns ma ...
s and philosophy
Philosophy (from , ) is the systematized study of general and fundamental questions, such as those about existence, reason, knowledge, values, mind, and language. Such questions are often posed as problems to be studied or resolved. Some ...
depend on unconscious skills that derive from the body, and that conscious symbol manipulation is only a small part of our intelligence.
See also
* Artificial intelligence, situated approach
In artificial intelligence research, the situated approach builds agents that are designed to behave effectively successfully in their environment. This requires designing AI "from the bottom-up" by focussing on the basic perceptual and motor sk ...
* Artificial philosophy
Artificial philosophy is a philosophical branch conceived by author Louis Molnarhttps://www.researchgate.net/publication/267156955_A_Step_Beyond_AI_Artificial_Philosophy "article: Frontiers in Artificial Intelligence and Applications", ResearchGat ...
Notes
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{{DEFAULTSORT:Physical Symbol System
Cognitive science
Philosophy of artificial intelligence
Formal systems
Cognitive modeling