Physical Symbol System (PSS) | Cognitive Science | BSc.CSIT (TU) | Fourth Semester

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physical symbol systemPhysical Symbol System (PSS)
Subject: Cognitive Science | BSc.CSIT (TU)
Fourth Semester | Tribhuvan University

Physical Symbol System (PSS)
The physical symbol system hypothesis (PSSH), first formulated by Newell and Simon, states that “a physical symbol system [such as a digital computer, for example] has the necessary and sufficient means for intelligent action.” The hypothesis implies that computers, when we provide them with the appropriate symbol-processing programs, will be capable of intelligent action. It also implies, as Newell and Simon wrote, that “the symbolic behavior of man arises because he has the characteristics of a physical symbol system.”

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).

A PSS consists of

  • Symbols – set of entities that are physical patterns
  • Symbol Structures – number of instances/tokens related in some physical way
  • Processes – operate on these expressions to produce other expressions

A natural question to ask about symbols and representation is what is a symbol? Allen Newell considered this question in Unified Theories of Cognition. He differentiated between symbols (the phenomena in the abstract) and tokens (their physical instantiations). Tokens “stood for” some larger concept. They could be manipulated locally until the information in the larger concept was needed, when local processing would have to stop and access the distal site where the information was stored. The distal information may itself be symbolically encoded, potentially leading to a graph of distal accesses for information.

Newell defined symbol systems according to their characteristics. Firstly, they may form a universal computational system. They have

  • memory to contain the distal symbol information,
  • symbols to provide a pattern to match or index distal information,
  • operations to manipulate symbols,
  • interpretation to allow symbols to specify operations, and,
  • capacities for:
    1. sufficient memory,
    2. composability (that the operators may make any symbol structure),
    3. interpretability (that symbol structures be able to encode any meaningful arrangement of operations).

Finally, Newell defined symbolic architectures as the fixed structure that realizes a symbol system. The fixity implies that the behavior of structures on top of it (i.e. “programs”) mainly depend upon the details of the symbols, operations and interpretations at the symbol system level, not upon how the symbol system (and its components) are implemented. How well this ideal hold is a measure of the strength of that level.

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