Physical Symbol Systems

Summer 2011
Monday, 07/18
Blade Runner
• Was Deckard a replicant?
Blade Runner
• Is there a characteristic feature that makes us
human, as opposed to a machine or a
Blade Runner
• What do you think of the "Voight-Kampff" test? Is it a
better measure of humanity (or of the human kind of
cognition) than the Turing test? Is emotion a better
mark of being human than reason? Are emotion and
reason even separable?
Blade Runner
• Could machines that don’t initially have
emotions develop emotions on their own,
without being explicitly programmed to
have/develop them?
Physical Symbol Systems
• A physical device that contains a set of interpretable and
combinable items (symbols) and a set of processes that can
operate on the items.
• A kind of automatic formal system.
• Symbol = stable entities that are capable of semantic
interpretation, that can participate in processes of internal
manipulation, e.g. copying, erasing, conjoining, and that
can be organized so as to preserve semantic sense.
• Must be located in a wider web of real-world items and
• A symbolic expression designates an object if “given the
expression, the system can either affect the object itself or
behave in ways depending on the object.”
Physical Symbol Systems
• The Physical Symbol System Hypothesis: A
physical symbol system has the necessary and
sufficient means for general intelligent action.
• Sufficient, since any such system “of sufficient
size” can always be programmed so as to support
intelligent behavior.
• Necessary, since nothing can be intelligent unless
it is an instance of a physical-symbol system
• This is an empirical hypothesis. All cases of
intelligent action will, as a matter of scientific
fact, turn out to be produced by a PSS.
Symbol Systems and the Brain
• The working of the individual neurons in the brain
may not be important in understanding the mind
as a physical symbol system.
• What matters is the operation of the “virtual
machine”, that can be realized in all sorts of ways.
• The role of symbols may be occupied by higherlevel brain processes/structures. It may even be
that different types of brain processes/structures
play the role of the same symbol at different times.
• You should not expect to find things like EI, EII,
EIILIIL (etc) in the brain! Just some physical thing
that plays the role of these symbols.
Semantically Transparent Systems
Systems whose computational operations are
defined over “familiar symbolic elements”
(Clark). For example:
• A chess-playing program that applies
procedures to symbols for rook, king,
• A sentence parser that uses symbols for noun,
verb, subject.
• A program for reasoning about liquids that has
symbols for liquid, flow, edge.
Why Treat Thought as Symbol
• Thinkers are physical devices whose behavior patterns
are reason respecting.
• A pedestrian witnesses a car crash, runs to a
telephone, and punches out 911.
• Common sense Psychology makes sense of all this at a
stroke by depicting the agent as seeing a crash and
wanting to get help.
• The simplest scientific explanation is that the agent’s
brain contains symbols that represent the event as a
car crash and that the computational state-transitions
occurring inside the system then lead to new sets of
states (more symbols) whose proper interpretation is,
e.g. “seek help”, “find a telephone”, and so on.
Why Treat Thought as Symbol
• The thought “it is raining” often leads to the
thought “let’s go indoors”.
• Many of our thoughts are related to our other
thoughts in virtue of their meaning in this way.
• One explanation of such rational thoughttransitions appeals to general syntactic rules
that manipulate semantic representations in a
way that preserves semantic sense. (think of
the PQ— system again…)
Why Treat Thought as Symbol
• We have the capacity to understand an infinite
range of sentences and to produce an infinite
range of thoughts.
• “Billy left his tricycle on the moon”.
• What could explain our capacity to
understand infinite new sentences like this
and to entertain infinite thoughts of this sort?
Why Treat Thought as Symbol
• The ability to entertain certain thoughts is
intrinsically connected to the ability to entertain
certain other thoughts.
• We don't find speakers who know how to express
in their native language the fact that John loves
the girl but not the fact that the girl loves John.
This is appears true for expressions of any n-place
relation, e.g. Eli (x) gave his paper (y) to Jon (z).
• One explanation appeals to general rules for
conjoining (interpreted) symbols in the language.
• We’ll talk a lot more about this next week…
Examples: Story Understanding
• A computer program that deploys scripts.
• The scripts uses a symbolic event description
language to encode background information
about certain kinds of situations, e.g. restaurant
• Takes as input a short story: “Jack goes into the
restaurant, orders a hamburger, sits down. Later,
he leaves after tipping the waiters.” You can then
ask: “Did Jack eat the hamburger?” and the
computer answers “yes” by applying the script.
Examples: SOAR
• ongoing project to implement general
intelligence by computational means.
• Uses symbol processing architecture.
• All long-term knowledge is stored in a format
called production memory.
• Knowledge is encoded in the form of
condition-action structures: “If such-and-such
is the case, then do so and so”.
Examples: SOAR
• When it encounters a problem, it transfers all
potentially relevant knowledge into a workingmemory buffer.
• A decision procedure then selects an action to
perform on the basis of relative desirability.
• SOAR is able to work towards a distant goal by
creating and attempting to resolve sub-goals that
reduce distance between current state and
overall solution.
• Learns by “chunking”.
Research Program (GOFAI)
• Design a program that can solve problems and
interact with the environment in human ways.
• If such a program is found, a good case can be
made that it’s actually implemented by
human brains.
• We can thus study minds directly (by studying
the software) without worrying about the
messy details of the brain.

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