representations view markdown

Some notes on knowledge representation based on Berkeley's CS 188 course and "Artificial Intelligence" Russel & Norvig 3rd Edition

intro

AI - field of study which studies the goal of creating intelligence
- intelligent agent - system that perceives its environment and takes actions that maximize its chances of success
expert task examples - medical diagnosis, equipment repair, computer configuration, financial planning

formal systems - use axioms and formal logic
ontologies - structuring knowledge in graph form
statistical methods

turing test - is human mind deterministic { turing1950computing }
chinese room argument - rebuts turing test { cite searle1980minds }
china brain - what if different people hit buttons to fire individual neurons
Polanyi’s paradox: “we can know more than we can tell”

symbol search

computer science - empirical inquiry

symbols and physical symbol systems

intelligence requires the ability to store and manipulate symbols
laws of qualitative structure
- cell doctrine in biology
- plate tectonics in geology
- germ theory of disease
- doctrine of atomism
“physical”
1. obey laws of physics
2. not restricted to human systems
  - designation - then given the expression, the system can affect the object
  - interpretation - expression designates a process

heuristic searching

symbol systems solve problems with heuristic search
Heuristic Search Hypothesis - solutions are represented as symbol structures. A physical symbol system exercises its intelligence in problem solving by search–that is, by generating and progressively modifying symbol structures until it produces a solution structure
- from { cite newell1976computer }
there are practical limitations on how fast computers can search
To state a problem is to designate
1. a test for a class of symbol structures (solutions of the problem)
2. a generator of symbol structures (potential solutions).
To solve a problem is to generate a structure, using (2), that satisfies the test of (1).
searching is generally in a tree-form

knowledge representation

physical symbol system hypothesis - a physical symbol system has the necessary and sufficient means for general intelligent action
- computers and minds are both physical symbol systems
- symbol - meaningful pattern that can be manipulated
- symbol system - creates, modifies, destroys symbols
want to represent
1. meta-knowledge - knowledge about what we know
2. objects - facts
3. performance - knowledge about how to do things
4. events - actions
two levels
1. knowledge level - where facts are described
2. symbol level - lower
properties
1. representational adequacy - ability to represent
2. inferential adequacy
3. inferential efficiency
4. acquisitional efficiency - acquire new information
two views of knowledge
1. logic
  - a logic is a language with concrete rules
  - syntax - rules for constructing legal logic
  - semantics - how we interpret / read
  - assigns a meaning - multi-valued logic - not just booleans - higher-order logic - functions / predicates are also objects - multi-valued logics - more than 2 truth values
  - fuzzy logic - uses probabilities rather than booleans - match-resolve-act cycle
2. associationist
  - knowledge based on observation
  - semantic networks - objects and relationships between them - like is a, can, has
  - graphical representation
  - equivalent to logical statements
  - ex. nlp - conceptual dependency theory - sentences with same meaning have same graphs
  - frame representations - semantic networks where nodes have structure
    - ex. each frame has age, height, weight, …
  - when agent faces new situation - slots can be filled in, may trigger actions / retrieval of other frames
  - inheritance of properties between frames
  - frames can contain relationships and procedures to carry out after various slots filled
statistical
- distributed - usually different from sparse code (sparser generally less robust)
  - opposite of sparse code = dense code
  - have to check multiple indexes
  - penti’s work: distributed
  - usually want these to be robust
  - nlp is main place where unsupervised pretraining widely used
- hierarchical
- good representations - linearly separable
- representation that factors
- information bottleneck method: want simple representation that keeps class but throws away lots of extraneous info

expert systems

expert system - program that contains some of the subject-specific knowledge of one or more human experts.
problems
1. planning
2. monitoring
3. instruction
4. control
need lots of knowledge to be intelligent
rule-based architecture - condition-action rules & database of facts
acquire new facts
- from human operator
- interacting with environment directly
forward chaining
- until special HALT symbol in DB, keep following logical rule, add result to DB
conflict resolution - which rule to apply when many choices available
pattern matching - logic in the if statements
backward chaining - check if something is true
- check database
- check if on the right side of any facts
CLIPS - expert system shell
- define rules and functions…
explanation subsystem - provide explanation of reasoning that led to conclusion
people
1. knowledge engineer - computer scientist who designs / implements ai
2. domain expert - has domain knowledge
  1. user interface
  2. knowledge engineering - art of designing and building expert systems
    - determine characteristics of problem
    - automatic knowledge-acquisition - set of techniques for gaining new knowledge
      - ex. parse Wikipedia
      - crowdsourcing
creating an expert system can be very hard
- only useful when expert isn’t available, problem uses symbolic reasoning, problem is well-structured
MYCIN - one of first successful expert systems { cite shortliffe2012computer }
- Stanford in 1970s
- used backward chaining but would ask patient questions - sometimes too many questions
advantages
- can explain reasoning
- can free up human experts to deal with rare problems

Godel, Escher, Bach

Douglas Hofstadter, 1979

music

canons - repeat w/ subtle changes (e.g. pitch shift)
fugue - repeat w/ more substantial changes

ai

essential abilities - can we do these things unsupervised?
- to recognize the relative importance of different elements of a situation
- to find similarities between situations despite differences which may separate them;
- to draw distinctions between situations despite similarities which may link them
- to synthesize new concepts by taking old concepts and putting them together in new ways
intelligence consists of rules at different levels
- “just plain” rules - like reflexes which respond to stereotyped situations
- metarules - when situations are mixtures of steretoyped situations, requires rules for deciding which “just plain” rules to apply
- rules for inventing new rules - when situations can’t be classified
  - rules may have to change themselves
messages - comparison of DNA to a jukebox
- where is info stored? records? buttons? smashed buttons?
- what could constitute a Rosetta stone for DNA codes?
- 3 parts
  - frame - tells you that this is a message
  - outer - tells you how to read a message (e.g. language, style)
  - inner - actual content
- if decoding is universal, we might call the outer message (e.g. the trigger) the message
memory - same bits can be used for different things - part of each message specifies the instruction type

brain

intelligence involves a calculus of descriptions = symbols
- symbols represent both classes + instances (maybe both depending on amount of activation / context) - def need some context
- can have links to other symbols (+priors on these)
- top-down logical structure??
- different ways to combine symbols get blurry
  - symbols can be learned to branch, merge
- can harness temporal firing rates to encode more
- can grow incrementally (greedily)
analogy of thoughts as trips on a (poorly fleshed out) map

interpretation

ex. top is decimal expansion of the sum of the second ($\pi/4$)
- 7, 8, 5, 3, 9, 8, 1, 6, …
- 1, -1/3, +1/5, -1/7, +1/9, -1/11…

math / logic

godel’s thm - limitation of any formal axiomatic system: cannot make a program to find a complete + consistent set of axioms
church-turing thesis - a function on the natural numbers can be calculated by an effective method if and only if it is computable by a Turing machine
- no system can do computation which cannot be broken down into simple elements
decision procedure - decides whether something is a theorem - must terminate
we can think of theorems as strings in a formal system
interpretation - correspondence between symbols and words
- ideally, these are meaningful isomorphisms between codes and reality
- not all interpretations imply meaningful (or valid) corresponding codes
- there might be multiple, equally valid interpretations
- consistency depends on interpretation:
- consistency - when every theorem, upon interpretation, comes out true (in some imaginable world)
- completeness - when all statements which are true (in some imageinable world), and which can be expressed as well-formed strings of the system, are theorems
slightly different axioms lead to elliptical/hyperbolic geometry instead of Euclidean geometry
godel numbering - can replace all symbols w/ numbers and all typographic rules w/ arithmetic rules
2 key idesas
- strings can speak about other strings
- self-scrutiny can be entire concentrated into a single string
every aspect of thinking can be viewed as a high-level description of a system which, on a low level, is governed by simple, even formal rules

causality

what counterfactuals are the most realistic
- different things are stable at different levels

biology

dna -> rna -> proteins = sequence of amino acids
- folds w/ valrious levels of structure (like music)
self-rep - what counts?
- quine? instructions on jukebox? human reproduction?

intro

symbol search

symbols and physical symbol systems

heuristic searching

knowledge representation

expert systems

Godel, Escher, Bach

meta

music

ai

brain

interpretation

math / logic

causality

biology