Chapter 7: Logical Agents

first 3 things agent program does when called

1. TELLs knowledge base what it perceives 2. ASKs knowledge base what action to perform 3. TELLs knowledge base what action was chosen and executes

declarative approach to system building

1. starting w/empty knowledge base, designer can TELL sentences one by one until agent knows how to operate in its environment

how to query known knowledge base?

TELL and ASK operations

axiom

a sentence taken as given without being derived from other sentences

How do we know that the knowledge base is true in the real world?

agent's sensors create the connection.

knowledge-based agents

agents that make decisions by processes of reasoning that rely on internal representationss of knowlege composed of a knowledge base and an inference mechanism. Operates by storing sentences about the world in its knowledge base, using that inference mechanism to infer new sentences, and using these sentences to decide which action to take

sound or truth-preserving algortihm

an inference algorithm that derives only entailed sentences. Highly desirable.

grounding

connection between logical reasoning processes and real environment in which the agent exists.

atomic sentences

consist of a single proposition symbol, each of which stands for a proposition that can be true or false.

complex sentences

constructed from simpler sentences using parentheses and logical connectives

representation language

defined by its syntax, which specifies the structure of sentences, and its semantics which defines the truth of each sentence in each possible world or model

syntax of propositional logic?

defines allowable sentences.

complete inference algorithms

derive ALL sentences that are entailed

sound inference algorithms

derive ONLY sentences that are entailed

inference

deriving new sentences from old.

locality

each action changes no more than a small number of an environment's fluents

procedural approach to system building

encodes ddesired behaviors directly as program code.

frame axioms

explicitly assert all propositions that remain the same

What does SATPlan do?

finds models for a sentence containing the initial state, the goal, the successor-state axioms, the precondition axioms, and the action exclusion axioms.

logical entailment between sentences

idea that a sentence follows logically from another sentence a entails b iff, in every model in which a is true, b is also true

truth

if a sentence a is true in model m, we say that m satisfies a or sometimes m is a model of a

when is an inference algorithm complete?

if it can derive any sentence that is entailed

hybrid agent

maintains and updates a knowledge base as well as a current plan 1. initial kb contains the atemporal axioms - those that don't depend on the time step. 2. At each new time step, the new percept sequence is added along with all axioms that depend on the time, such as successor-state axioms. 3. then agent uses logical inference by ASKing questions of the kb to work out what to do next

what is a model?

mathematical abstractions, each of which simply fixes the truth or falsehood or a relevant sentence. Formally, possible models are all possible assignments of real numbers to variables.

semantics

meaning of sentences. Defines truth of each sentence with respect to each possible world.

inferential frame problem

problem of projectign forward the results of a t step plan of action in time O(kt) rather than O(nt)

state estimation

process of updating the belief state as new percepts arrive

belief state

representation of the set of all possible current states of the world.

knowledge base

set of sentences expressed in a knowledge representation language that represents some assertion about the world

syntax of representation languagge

specifies all the sentences that are well formed.

effect axioms

specify the outcome of an action at the next time step.

precondition axioms

state that an action occurrence requires the preconditions to be satisfied

atemporal variables

symbols associated with permanent aspects of the world

fluent

used to describe aspect of the world that changes