Propositional Logic Intro, Syntax

1
Propositional Logic Intro, Syntax Computer
Science cpsc322, Lecture 19 (Textbook Chpt
5.0-5.2) February, 25, 2008
2
Lecture Overview

• Recap (detailed examples of planning as CSP)
• Logic Intro
• Propositional Definite Clause Logic Syntax

3
Planning as a CSP

• An alternative approach to planning is to set up
  a planning problem as a CSP!
• We simply reformulate a STRIPS model as a set of
  variables and constraints
• Once this is done we can even express additional
  aspects of our problem (as additional
  constraints)

4
Planning as a CSP Variables

• To do this, we need to unroll the plan for a
  fixed number of steps
• this is called the horizon
• To do this with a horizon of k
• construct a variable for each feature at each
  time step from 0 to k
• construct a boolean variable for each action at
  each time step from 0 to k - 1.

5
Delivery Robot ExampleActions

• The robots actions are
• move clockwise (mc ),
• move anti-clockwise (mac )
• PUC - Rob picks up coffee
• must be at the coffee shop, and must not have
  coffee
• DelC - Rob delivers coffee
• must be at the office, and must have coffee
• PUM - Rob picks up mail
• must be in the mail room, and mail must be
  waiting
• DelM - Rob delivers mail
• must be at the office and have mail

6
Robot Example 1 one layer
(T,F)
(off, cs, mr, lab)
(off, cs, mr, lab)
mac0
(T,F)
(T,F)
(T,F)
(T,F)
7
Robot Example 2 one layer
(T,F)
(off, cs, mr, lab)
(off, cs, mr, lab)
mac0
(T,F)
(T,F)
(T,F)
(T,F)
8
Lecture Overview

• Recap (detailed examples planning as CSP)
• Logic Intro
• Propositional Definite Clause Logic Syntax

9
Agents acting in an environment
Representation Reasoning
Representation Reasoning
10
Big Picture what is coming next

• Environment

Stochastic
Deterministic
Search
Single Action
Constraint Satisfaction (CSPs)
Decision
Logics
Search
Sequence of Actions
Constraint Satisfaction (CSPs)
Planning
11
Logic A more general framework for
representation reasoning

• Let's now think about how to represent a world
  about which we have only partial (but certain)
  information
• What do we need to represent?

12
Why Logics?

• Natural to express knowledge about the world
• (more natural than a flat set of variables
  constraints)
• Every 322 student will pass the midterm
• It is easy to incrementally add knowledge
• It is easy to check and debug knowledge
• Provide language for asking complex queries
• Well understood formal properties

13
Propositional Logic

• We will study the simplest form of Logic
  Propositional
• The primitive elements are propositions
  Boolean variables that can be true, false
• The goal is to illustrate the basic ideas
• This is a starting point for more complex logics
  (e.g., first-order logic)
• Boolean nature can be exploited for efficiency.

14
Logics in practice

• Agent is told (perceives) some facts about the
  world
• Agent is told (already knows / learns) how the
  world works
• Agent can answer yes/no questions about whether
  other facts must be true

15
Using Logics in designing an Agent

• Begin with a task domain.
• Distinguish those things you want to talk about
  (the ontology).
• Choose symbols in the computer to denote
  propositions
• Tell the system knowledge about the domain.
• Ask the system whether new statements about the
  domain are true or false.

16
Electrical Environment
/down
/ up
17
Propositional logic Complete Language

• The proposition symbols p1, p2 etc are
  sentences
• If S is a sentence, ?S is a sentence (negation)
• If S1 and S2 are sentences, S1 ? S2 is a sentence
  (conjunction)
• If S1 and S2 are sentences, S1 ? S2 is a sentence
  (disjunction)
• If S1 and S2 are sentences, S1 ? S2 is a sentence
  (implication)
• If S1 and S2 are sentences, S1 ? S2 is a sentence
  (biconditional)

18
Propositional Definite Clauses

• Propositional Definite Clauses our first logical
  representation and reasoning system.
• (very simple!)
• Only two kinds of statements
• that a proposition is true
• that a proposition is true if one or more other
  propositions are true
• Why still useful?
• Adequate in many domains (with some adjustments)
• Reasoning steps easy to follow by humans
• Inference linear in size of your set of
  statements
• Similar formalisms used in cognitive architectures

19
Representation and Reasoning System

• Definition (RSS)
• A Representation and Reasoning System (RRS) is
  made up of
• syntax specifies the symbols used, and how they
  can be combined to form legal sentences
• semantics specifies the meaning of the symbols
• reasoning theory or proof procedure a
  specification of how an answer can be produced.

20
Lecture Overview

• Recap (detailed examples planning as CSP)
• Logic Intro
• Propositional Definite Clause Logic Syntax

21
Propositional Definite Clauses Syntax
Definition (atom) An atom is a symbol starting
with a lower case letter
Definition (body) A body is an atom or is of the
form b1 ? b2 where b1 and b2 are bodies.
Definition (definite clause) A definite clause is
an atom or is a rule of the form h ? b where h
is an atom and b is a body. (Read this as h
if b.'')
Definition (atom) A knowledge base is a set of
definite clauses
22
PDC Syntax Example

• The following are syntactically correct
  statements in our language
• ai_is_fun
• ai_is_fun ? get_good_grade
• ai_is_fun ? get_good_grade ? not_too_much_work
• ai_is_fun ? get_good_grade ? not_too_much_work ?
  prof_can_operate_laptop
• The following statements are syntactically
  incorrect
• ai_is_fun ? ai_is_boring
• ai_is_fun ? relaxing_term ? get_good_grade ?
  not_too_much_work

Do any of these statements mean anything?
Syntax doesn't answer this question.
23
Next class

• Definite clauses Semantics and Proofs (textbook
  5.3)