BDI-Agent

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Belief Desire Intention

Agents

FromReasoning about Rational AgentsBy Michael Wooldridge

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Rational Agents

Properties of AgentsSituated or embodied in some environment

Set of possible actions that modify environment

Autonomy -- makes independent decisions

Proactivness -- able to exhibit goal directed behavior

Reactivity -- responsive to changes in environment

Social Ability -- interact with other agents(negotiation and cooperation)

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BDI Model of rational agency

Beliefs -- information the agent has about

world

Desires -- states of affairs that the agent, inan ideal world, would wish to be brought

about

Intentions -- desires that the agent hascommitted to achieving

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BDI Model

The intuition is that the agent will not, ingeneral, be able to achieve all its desires.

Therefore an agent must fix upon some

subset of its desires and commit resources to

achieving them. These chosen desires are

intentions.

Developed by Michael Bratman

Intention based theory of practical reasoning

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Reasoning in Humans

Practical reasoning is reasoning directed toward actions the process of figuring out what to do

Theoretical reasoning is reasoning directed towardbeliefs

Ex. All men are mortal AND Socrates is a man -> __ Practical reasoning has 2 activities

(Deliberation) Deciding what state of affairs we want toachieve

(Means end reasoning) Deciding how we want to achievethese state of affairs

Ex. When a person graduated from the university .

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Practical Reasoning

A straight forward process?

Some complications

Deliberation and means end reasoning are

computational processes

Resource bounds, time constraints

Two implications

Good performance requires efficient use of resources

Cannot deliberate indefinitely

Must commit to a state of affairs (called intention)

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Intentions in practical reasoning

Use of the term in ordinary speechCharacterize actions (not accidental)

I might intentionally push someone under a train,

with the intention of killing them.

Characterizestates of mind

I might have the intention this morning of pushingsomeone under a train this afternoon.

Future directed intentions are states of mind that aredirected toward a future state of affairs

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Intentions drive means end reasoningA reasonable attempt to achieve is made

Involves deciding how to achieve

Basketball example Intentions persist

I will not give up without good reason

Drop when its achieved, impossible or thereason for intention is no longer true.

Academic example

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Constrain future practical reasoning If I hold an intention I will not entertain options that are

inconsistent with that intention

Influence beliefs upon which future practical reasoningis based

If I adopt an intention, then I can plan for the future on theassumption that I will achieve the intention. For if I intend to

achieve some state of affairs while simultaneously believingthat I will not achieve it, then I am being irrational


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Agent control loop Version 1

1. While true

2. Observe the world

3. Update internal world model

4. Deliberate about what intention to achieve next

5. Use means end reasoning to get a plan for the

intention

2. Execute plan3. End while

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Observe-Think-Act loop[2]

1. observe the world;

2. interpret the observations (if needed):

diagnose (includes testing);

learn (includes testing);

3. select a goal;

4. plan;

5. execute part of the plan.

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Plans

Plans are recipes for achieving intentions A tuple of

Pre conditions -- circumstances under which it

is applicable

Post condition -- defines what state of affairs

the plan achieves

Body -- a sequence of actions

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Agent control loop version 2

1. B := B0 // initial beliefs2. While true do

3. get next precept p

4. B := brf(B, p) //belief revision function

5. I := deliberate(B)

6. Pi := plan(B,I)

7. execute(Pi)

8. End while

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The Deliberation Process

Option generationAgent generates a set of possible alternatives

Filtering

Agent chooses between competing alternatives,and commits to achieving them

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Agent control loop 3

1. B := B0 // initial beliefs2. I := I0 // initial intentions

3. While true do


5. B := brf(B, p)

6. D := options(B,I) // D - desires

7. I := filter(B) // I - intentions

8. Pi := plan(B,I)

9. execute(Pi)

10. End while

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Commitment to intentions

An agent is committed to an intention How committed should an agent be?

How long should it persist?

Under what conditions should a intentionvanish?

Commitment strategyis used to determine

when and how to drop to drop intentions

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Commitment Strategies

Blind Commitment

Single minded

Open minded

Maintain

Until intention has been

achieved

Until Intention achieved

or no longer possible

While believed possible

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Agent control loop 4 -- introduce reactivity, replan1. B := B0 I := I0 // initial beliefs and intentions

2. While true do


4. B := brf(B, p)

5. D := options(B,I)

6. I := filter(B)

7. Pi := plan(B,I)

8. While not empty(Pi) do9. a := hd(Pi)

10. execute(a)

11. Pi = tail(Pi)


13. B := brf(B,p)14. if not sound(Pi, I, B) then

15. Pi := plan(B,I)

16. end if

17. End while

18. End while

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Agent Control loop 5 -- can drop intentions

1. B := B0 I := I0 // initial beliefs and intentions

2. While true do

3. get next precept p4. B := brf(B, p)


6. I := filter(B)

7. Pi := plan(B,I)

8. While not (empty(Pi) or succeeded(I,B) or impossible(I,B)) do9. a := hd(Pi)

10. execute(a)

11. Pi = tail(Pi)


13. B := brf(B,p)

14. if not sound(Pi, I, B) then

15. Pi := plan(B,I)

16. end if

17. End while

18. End while

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Commitment to means and ends

Intentions -- ends Plan -- means

Replan if plan is no longer sound given beliefs

Beliefs are updated after execution each action

Reconsiders plan after each iteration but does

not reconsider intentions

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Intention Reconsideration

Reconsiders whenCompletely executed plan

Believes it has achieved current intentions

Believes current intentions are no longer possible Does not allow agent to exploit serendipity

reconsideration of intention during theexecution of the plan

A t t l l 6 ti

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Agent control loop 6 -- cautious1. B := B0 I := I0 // initial beliefs and intentions

2. While true do


4. B := brf(B, p)5. D := options(B,I)

6. I := filter(B)

7. Pi := plan(B,I)

8. While not (empty(Pi) or succeeded(I,B) or impossible(I,B)) do

9. a := hd(Pi)

10. execute(a)

11. Pi = tail(Pi)


13. B := brf(B,p)


15. I := filter(B, D, I)


17. Pi := plan(B,I) end if

18. End while19. End while

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Reconsideration of intentions

How often to recondsider your intentions? How to charactarize situtations in which

reconsideration would take plan?

Reconsideration requires resources

How fast is the environment changing?

A t t l l 7 b ld / ti t

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Agent control loop 7 -- bold / cautious agent1. B := B0 I := I0 // initial beliefs and intentions

2. While true do


4. B := brf(B, p)5. D := options(B,I)

6. I := filter(B)

7. Pi := plan(B,I)

8. While not (empty(Pi) or succeeded(I,B) or impossible(I,B)) do

9. a := hd(Pi)

10. execute(a)

11. Pi = tail(Pi)


13. B := brf(B,p)

14. if reconsider(I,B) then

15. D := options(B,I)16. I := filter(B, D, I) end if


18. Pi := plan(B,I) end if

19. End while

20. End while

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BDI model implementation Procedural reasoning system (PRS)

Beliefs -- > prolog like facts

Desires and intentions are realized from plan library

Plans achieve some state of affairs

A plan has body and invocation condition

Invoked plans are desires

The agent picks one desires and puts it on executionstack

Execution stack are intentions

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Bibliography

Wooldridge, 1999, Reasoning about Rational Agents

Balduccini, 2005,Answer set Base Design of Highly Autonomous, Rational

Agents

Documents

BDI-Agent