Artificial Minds - Spring 2009

Class notes for Monday, February 9 (week 4)

===============================================================================
DISCLAIMER:  These are my own working lecture notes, which I'm posting here in
case they're useful.  I'll try to post my notes on a more or less regular
basis (assuming that I have some to post), but I make no guarantees as to
their completeness or comprehensibility.  They're also not too pretty, unless
you're a fan of plain text and ASCII art, so I'd recommend using them as a
supplement rather than a replacement for your own note-taking.  In other words,
you're still responsible for the material we cover in class, whether or not
that material is included here.
===============================================================================

Birth of AI: 1956 Dartmouth conference
- John McCarthy at Dartmouth (organizer)
- Marvin Minsky at Harvard (organizer)
- Nathaniel Rochester at IBM (sponsor)
- Claude Shannon at Bell Labs (sponsor)
- Ray Solomonoff at MIT (working on symbolic theories of logic)
- Oliver Selfridge at MIT (working on machine pattern recognition)
- Arthur Samuel at IBM (working on checkers learning program)
- Trenchard More at Princeton (grad student)
- Herbert Simon at Carnegie Tech (professor of industrial administration)
- Allen Newell at RAND Corporation
- for decades afterward, almost all significant advances in AI were made by the
  Dartmouth conference participants or their students
- John McCarthy is credited with the phrase "artificial intelligence", which
  defined the field's identity

--------------------------------------------------------------------------------
Early AI work by Newell & Simon
- Logic Theorist (1956)
- GPS (1957)

Logic Theorist
- proved theorems of logic
- early versions of the program were hand-simulated using index cards
- eventually proved 38 of the first 52 theorems in Principia Mathematica
- one proof was even more elegant than Russell and Whitehead's proof, to Newell
  & Simon's surprise
- they submitted a paper with the new proof to the Journal of Symbolic Logic,
  with the program as co-author (but the editor rejected it)
- "we invented a computer program capable of thinking non-numerically, and
  thereby solved the venerable mind/body problem, explaining how a system
  composed of matter can have the properties of mind" --Herbert Simon

General Problem Solver (GPS)
- Newell & Simon conducted psychological experiments to find out how people
  solved logic puzzles by having them describe their reasoning processes out
  loud
- from these experiments they extracted general principles of problem-solving
  and designed them into GPS
- their work defined the "state space search" approach to problem-solving for
  many years in AI
- problem spaces, operators, search trees, goals/subgoals, heuristic decisions,
  backtracking, means-ends analysis
- "weak methods": independent of the particular task being solved
- applied to many tasks: logic puzzles, symbolic integration, code-breaking

--------------------------------------------------------------------------------
Examples of state-space search problems

Missionaries and Cannibals problem
  - 3 missionaries, 3 cannibals, 1 boat
  - 1 or 2 people at a time can cross the river in the boat
  - get everyone safely to the other side while ensuring that
    cannibals never outnumber missionaries in the same place
  - states: number of missionaries and cannibals on each side of river,
	    location of boat
  - operators: move1M, move1C, move2M, move2C, move1M1C
  - initial state: everyone on left side, boat on left
  - goal state: everyone on right side, boat on right

  Solution (11 steps):

     MMMCCC  \__/.....  (initial state)

     2 cannibals cross the river (move2C)
     MMMC  .....\__/  CC

     1 cannibal comes back (move1C)
     MMMCC  \__/.....  C

     2 cannibals cross the river (move2C)
     MMM  .....\__/  CCC

     1 cannibal comes back (move1C)
     MMMC  \__/.....  CC

     2 missionaries cross the river (move2M)
     MC  .....\__/  MMCC

     1 missionary and 1 cannibal come back (move1M1C)
     MMCC  \__/.....  MC

     2 missionaries cross the river (move2M)
     CC  .....\__/  MMMC

     1 cannibal comes back (move1C)
     CCC  \__/.....  MMM

     2 cannibals cross the river (move2C)
     C  .....\__/  MMMCC

     1 missionary comes back (move1M)
     MC  \__/.....  MMCC

     1 missionary and 1 cannibal cross the river (move1M1C)
       .....\__/  MMMCCC

     Goal state reached!


Eight Puzzle problem
  - 3 x 3 grid of tiles with one space
  - unscramble tiles to reach a goal configuration
  - states: board configurations
  - operators: move tile adjacent to the blank up, down, left, or right

     3 5 2        3 5 2                 1 2 3
     4 6     =>   4   6   ... => ...    4 5 6
     8 7 1        8 7 1                 7 8

     initial                            goal
      state                             state