CMSC 671 Fall 2005Knowledge-Based AgentsA knowledge-based agentArchitecture of a knowledge-based agentThe Wumpus World environmentA typical Wumpus worldAgent in a Wumpus world: PerceptsWumpus actionsWumpus goalThe Wumpus agent’s first stepLaterLet’s play...Wumpuses onlineRepresentation, reasoning, and logicThe connection between sentences and factsSoundness and completenessLogic as a KR languageOntology and epistemologyNo independent access to the worldSummary1CMSC 671CMSC 671Fall 2005Fall 2005Class #9– Thursday, September 292Knowledge-Based AgentsChapter 7.1-7.3Some material adopted from notes by Andreas Geyer-Schulzand Chuck Dyer3A knowledge-based agent •A knowledge-based agent includes a knowledge base and an inference system.•A knowledge base is a set of representations of facts of the world. •Each individual representation is called a sentence. •The sentences are expressed in a knowledge representation language. •The agent operates as follows: 1. It TELLs the knowledge base what it perceives. 2. It ASKs the knowledge base what action it should perform. 3. It performs the chosen action.4Architecture of a knowledge-based agent•Knowledge Level. –The most abstract level: describe agent by saying what it knows. –Example: A taxi agent might know that the Golden Gate Bridge connects San Francisco with the Marin County. •Logical Level. –The level at which the knowledge is encoded into sentences. –Example: Links(GoldenGateBridge, SanFrancisco, MarinCounty). •Implementation Level. –The physical representation of the sentences in the logical level. –Example: ‘(links goldengatebridge sanfrancisco marincounty)5The Wumpus World environment •The Wumpus computer game•The agent explores a cave consisting of rooms connected by passageways. •Lurking somewhere in the cave is the Wumpus, a beast that eats any agent that enters its room. •Some rooms contain bottomless pits that trap any agent that wanders into the room. •Occasionally, there is a heap of gold in a room.•The goal is to collect the gold and exit the world without being eaten7A typical Wumpus world •The agent always starts in the field [1,1]. • The task of the agent is to find the gold, return to the field [1,1] and climb out of the cave.8Agent in a Wumpus world: Percepts •The agent perceives –a stench in the square containing the wumpus and in the adjacent squares (not diagonally) –a breeze in the squares adjacent to a pit–a glitter in the square where the gold is–a bump, if it walks into a wall–a woeful scream everywhere in the cave, if the wumpus is killed•The percepts are given as a five-symbol list. If there is a stench and a breeze, but no glitter, no bump, and no scream, the percept is [Stench, Breeze, None, None, None] •The agent cannot perceive its own location9Wumpus actions•go forward •turn right 90 degrees•turn left 90 degrees•grab: Pick up an object that is in the same square as the agent•shoot: Fire an arrow in a straight line in the direction the agent is facing. The arrow continues until it either hits and kills the wumpus or hits the outer wall. The agent has only one arrow, so only the first Shoot action has any effect •climb is used to leave the cave. This action is only effective in the start square•die: This action automatically and irretrievably happens if the agent enters a square with a pit or a live wumpus10Wumpus goalThe agent’s goal is to find the gold and bring it back to the start square as quickly as possible, without getting killed–1000 points reward for climbing out of the cave with the gold–1 point deducted for every action taken–10000 points penalty for getting killed11The Wumpus agent’s first step¬W¬W12Later¬W¬W¬P¬P¬W¬W13Let’s play...A14Wumpuses online•http://www.cs.berkeley.edu/~russell/code/doc/overview-AGENTS.html – Lisp version from Russell & Norvig•http://scv.bu.edu/cgi-bin/wcl –Web-based version you can play•http://codenautics.com/wumpus/ – downloadable Mac version15Representation, reasoning, and logic•The object of knowledge representation is to express knowledge in a computer-tractable form, so that agents can perform well. •A knowledge representation language is defined by: – its syntax, which defines all possible sequences of symbols that constitute sentences of the language. •Examples: Sentences in a book, bit patterns in computer memory. – its semantics, which determines the facts in the world to which the sentences refer. •Each sentence makes a claim about the world. •An agent is said to believe a sentence about the world.16The connection between sentences and factsSemantics maps sentences in logic to facts in the world.The property of one fact following from another is mirroredby the property of one sentence being entailed by another.17Soundness and completeness•A sound inference method derives only entailed sentences.•Analogous to the property of complet eness in search, a complete inference method can derive any sentence that is entailed.18Logic as a KR languagePropositional LogicFirst OrderHigher OrderModalFuzzyLogicMulti-valuedLogicProbabilisticLogicTemporalNon-monotonicLogic19Ontology and epistemology• Ontology is the study of what there is—an inventory of what exists. An ontological commitment is a commitment to an existence claim.• Epistemology is a major branch of philosophy that concerns the forms, nature, and preconditions of knowledge.20No independent access to the world •The reasoning agent often gets its knowledge about the facts of the world as a sequence of logical sentences and must draw conclusions only from them without independent access to the world.•Thus it is very important that the agent’s reasoning is sound!21Summary•Intelligent agents need knowledge about the world for making good decisions. •The knowledge of an agent is stored in a knowledge base in the form of sentences in a knowledge representation language. • A knowledge-based agent needs a knowledge base and an inference mechanism. It operates by storing sentences in its knowledge base, inferring new sentences with the inference mechanism, and using them to deduce which actions to take. •A representation language is defined by its syntax and semantics, which specify the structure of sentences and how they relate to the facts of the world. •The interpretation of a sentence is the fact to which it refers. If this fact is part of the actual world,