Physical Symbol System Hypothesis

1
Physical Symbol System Hypothesis

• A physical symbol system has the necessary and
  sufficient means for general intelligent action.
  (Newell and Simon, Computer Science as Empirical
  Enquiry...)
• What is a Physical Symbol System?

2
Five conditions

• 1. A symbol may be used to designate any
  expression (where designate means to affect
  causally or be affected by)
• 2. there exist expressions that designate every
  process of which the machine is capable
• 3. any expression can be created or modified in
  arbitrary ways
• 4. expressions can be stored without alteration
• 5. memory is unbounded.
• (One thing these give you is flexible, stored
  program computing.)

3
Modal v. Amodal symbols

• A modal symbol is one tied distinctively to a
  particular sensory mode or to a motor program
  (include bound sensory representations together).
  
• An amodal symbol is one that is not modal (these
  are typically characterized as arbitrary)

4
Perceptual Symbol Systems

• Human thought consists only of the manipulation
  or activation of sensory or motor states or
  stored records of them.
• On the view of Barsalou et. al, this can include
  binding units (conjunctive neurons), that cause
  the appropriate reactivation of the stored
  records.

5
Levels of Explanation

• We should distinguish between the
  cognitive-functional description of the models
  and the neural description.
• Being modal need not be a claim about cortex
  (that, e.g., anything appearing in sensory or
  motor cortex is automatically the physical form
  of a modal representation).

6
Tied distinctively to...

• What does this mean, then?
• Its partly a claim about processing that the
  way in which symbols are processed is influenced
  by the particular modality in which they appear.

7
Local v. global representation

• Global form assumption the same symbol
  represents a given feature (property) in
  connection with all relevant concepts. These are
  typically assumed to be amodal (i.e., not tied to
  any particular sensory modality).
• Local form assumption distinct symbols are used
  to represent the same property in connection with
  the various relevant concepts. (These are
  frequently assumed to be modal.)

8
Phenomena that might seem hard to for the
perceptual SS to explain

• Categorical inference. Inferences based on the
  category something belongs to or relation between
  categories
• Sam is a dog. All dogs are animals. All animals
  can die. Therefore, dogs can die. Therefore, Sam
  can die.
• Type-token distinction. We regularly distinguish
  between individuals (tokens) and the categories
  to which they belong (types).
• Abstraction Forming categories requires ignoring
  many properties of individual members.

9
Productivity and Compositionality

• Human language and thought are generative an
  open-ended set of sentences or thoughts can be
  generated from a relatively small number of rules
  and unstructured representations.
• The content of a thought or statement is a
  function of the content of its parts and
  combinatorial rules. (Even when its not, the
  meaning is often a function of theoretical
  factors.)

10
Conjunction detectors

• Function as simple markers that reactivate past
  records of sensory experience.
• Can code for similarities among past sensory
  experiences.
• This coding explains abstraction and can explain
  how the right combinations of sensory experiences
  get re-activated.

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• Reactivation of selected sensory records can
  account for productivity and compositional
  semantics.
• It also accounts for categorical inference and
  the type-token distinction.
• The particular sensory record is the token the
  type is a collection of aspects (common features)
  of particular records.

12
Status of conjunctive neurons

• Read from p. 88.
• Are higher-order conjunctive neurons amodal
  symbols?
• What determines what they activate and how the
  results will be read?