Lecture 4-1 CS250: Intro to AI/Lisp

Logical Reasoning I

Lecture 4-2

January 25th, 1999

CS250

Lecture 4-1 CS250: Intro to AI/Lisp

Announcements• Unix classes

• Office hours today

• TA (Office hours,Style)

• Quiz

• Midterm on Tuesday of 6th Week

• Project descriptions by email by end of week

• Lisp for Windows

Lecture 4-1 CS250: Intro to AI/Lisp

A Little Lisp

• What about progn?

• Validity checking?

Lecture 4-1 CS250: Intro to AI/Lisp

Designing an Interplanetary Explorer

• Goals for the explorer

• Actions we can take

• Situations that might arise

Lecture 4-1 CS250: Intro to AI/Lisp

Telling Your Computer about the World

• What you know

• How to go from what you know to what you don’t know

• Will Rogers

Lecture 4-1 CS250: Intro to AI/Lisp

Knowledge Base

Knowledge base:Database :: Knowledge:Data

• KB stores what the computer knows about the world

• Knowledge representation language– How we encode knowledge about the

world– Each bit of knowledge is a sentence

Lecture 4-1 CS250: Intro to AI/Lisp

Getting through to your computer

• KB Interaction– Tell: Add new sentences to the KB– Ask: Query what’s known (or what follows

from what is known)

Lecture 4-1 CS250: Intro to AI/Lisp

;;;; Main Functions on KBs: Tell, Retract, Ask-Each, ;;;; Ask, Ask-Pattern[s]

;;; First we define a very simple kind of knowledge base,;;; literal-kb, that just stores a list of literal sentences.

(defstructure literal-kb "A knowledge base that just stores a set of literal sentences." (sentences '()))

Tell-Ask.lisp I

Lecture 4-1 CS250: Intro to AI/Lisp

;;; There are three generic functions that operate on;;; knowledge bases, and that must be defined as methods;;; for each type of knowledge base: TELL, RETRACT, and;;; ASK-EACH. Here we show the implementation for literal-kb;;;; elsewhere you'll see implementations for propositional,;;; Horn, and FOL KBs.

(defmethod tell ((kb literal-kb) sentence) "Add the sentence to the knowledge base." (pushnew sentence (literal-kb-sentences kb) :test #'equal))

(defmethod retract ((kb literal-kb) sentence) "Remove the sentence from the knowledge base." (deletef sentence (literal-kb-sentences kb) :test #'equal))

(defmethod ask-each ((kb literal-kb) query fn) "For each proof of query, call fn on the substitution that the proof ends up with." (declare (special +no-bindings+)) (for each s in (literal-kb-sentences kb) do (when (equal s query) (funcall fn +no-bindings+))))

Tell-Ask.lisp II

Lecture 4-1 CS250: Intro to AI/Lisp

;;; There are three other ASK functions, defined below,;;; that are defined in terms of ASK-EACH. These are ;;; defined once and for all here (not for each kind ;;; of KB)."

(defun ask (kb query) "Ask if query sentence is true; return t or nil." (ask-each kb (logic query) #'(lambda (s) (declare (ignore s)) (RETURN-FROM ASK t))))

;;; Omitted pattern-matching ASK’s

Tell-Ask.lisp III

Lecture 4-1 CS250: Intro to AI/Lisp

Knowledge-Based Agents

Agents perceive the world around them Perceptions are recorded in the KB Actions are chosen based on the KB Results of actions are recorded

Lecture 4-1 CS250: Intro to AI/Lisp

Levels of Agents

• Knowledge level– What an agent knows– Planetary core samples must be taken at

least 100mm below the surface

• Logical level– Knowledge is encoded at this level– MinDepth(CoreSample,100)

• Implementation level– Inside the machine

Lecture 4-1 CS250: Intro to AI/Lisp

Building Knowledge Agents

• Lean on the inference mechanism– Tell agent what it needs to know

• Declarative– Declare the state of the world, and let ‘er

rip

• Adding learning– Reacting to percepts

Lecture 4-1 CS250: Intro to AI/Lisp

Separate Domain-Specific from the General

Lecture 4-1 CS250: Intro to AI/Lisp

Wumpus World

Lecture 4-1 CS250: Intro to AI/Lisp

Specifying the Wumpus World

• Percepts?

• Actions?

• Goals?

Lecture 4-1 CS250: Intro to AI/Lisp

Describing the Wumpus World

• Is the world…– Deterministic– Fully accessible– Static– Discrete

Lecture 4-1 CS250: Intro to AI/Lisp

One Environment is Easy

• If we know the environment well, can engineer it

• Range of environments?

Lecture 4-1 CS250: Intro to AI/Lisp

Exploring the world

Perceive: [None, None, None, None, None]

Perception: Stench, Breeze, Glitter, Bump, Scream

Lecture 4-1 CS250: Intro to AI/Lisp

Move Forward to 2,1

Lecture 4-1 CS250: Intro to AI/Lisp

Perception after One Move

Stench: None

Breeze: Yes

Glitter: None

Bump: None

Scream: None

Lecture 4-1 CS250: Intro to AI/Lisp

What Does the World Look Like?

Lecture 4-1 CS250: Intro to AI/Lisp

Knowledge Representation

• Not just computer readable…

…computer reasonable as well

• Syntax - Rules for building expressions

• Semantics - Relationship between facts in the world and sentences

• Examples?

Lecture 4-1 CS250: Intro to AI/Lisp

Entailment

• What follows from what

• Entailment is relationship among sentences– KB entails a

• “Follows” is a relationship among facts in the world

• Inference procedures that generate only entailed sentences is sound

Lecture 4-1 CS250: Intro to AI/Lisp

Logical Committment