Agent definition

1. Dictionary definition: agent (ay­gent) n. something that produces or is capable of producing an effect: an active or efficient cause. one who acts for or in the place of another by authority from him ... a means or instrument by which a guiding intelligence achieves a result. Agent definition

2. Computer Science definition: An agent is a computer system, situated in some environment, that is capable of flexible autonomous action in order to meet its design objectives. This definition embraces three key concepts: situatedness The agent receives sensory input from its environment and it can perform actions which change the environment in some way. autonomy flexibility Agent definition

3. Dictionary definitions: Autonomy : self­determined freedom, especially moral independence. Autonomous: self­governing, independent. Agent definition The system should be able to act without the direct intervention of humans (or other agents). The system should have control over its own actions and internal state. Example: Autonomous navigation Sometimes used in a stronger sense to mean systems that are capable of learning from experience. Autonomy

4. any process control system: monitor real­world environment and perform actions to modify it as conditions change (typically in real­time), simple ­ thermostats, very complex ­ nuclear reactor control systems. software deamons: monitor software environment and perform actions to modify the environment as conditions change, UNIX xbiff program, monitors a user's incoming mail and displays an icon when new mail is detected. However, these systems are not capable of flexible action in order to meet their design objectives. Examples of existing situated, autonomous computer systems

5. pro­active: agents should not simply act in response to their environment, they should be able to exhibit opportunistic, goal­directed behaviour and take the initiative where appropriate; social: agents should be able to interact, when appropriate, with other artificial agents and humans in order to complete their own problem solving and to help others with their activities. responsive: agents should perceive their environment and respond in a timely fashion to changes that occur in it; Agents may have other characteristics, e.g. mobility, adaptability, but those given here are the distinguishing features of an agent. Flexibility

6. Agents have their roots in traditional AI Also builds on contributions from other long established fields: object­oriented programming concurrent object­based systems human­computer interface design Historically AI researchers tended to focus on different components of intelligent behaviour, e.g. learning, reasoning, problem solving, vision understanding. The assumption seemed to be that progress was more likely to be made if these aspects of intelligent behaviour were studied in isolation. The road to intelligent agents

7. Phase 1: formal, structured problems with well defined boundaries (block worlds, game playing, symbol manipulation, reasoning, theorem proving) Combination to create integrated AI systems was assumed to be straightforward. Phase 2: expert systems; building on domain-specific knowledge for specialist problems Rule-based systems Phase 3: specialised areas such as vision, speech, natural language processing, robot control, data mining Mainly sensory data Intelligent agents seen currently as the main integrating force AI development stages

8. The right action is the one that makes the agent the most successful Need measures of success E.g. pick the most points, make the least number of moves, minimise power consumption, etc. Rationality depends on performance measures, prior knowledge, actions, event history For each possible event sequence, the rational agent should select an action that is expected to maximise its performance measure, given the evidence provided by the event sequence and the built-in knowledge the agent has. Important: rationality maximises expected performance, not actual (we cannot tell the future) Rational agents

9. Rational agents should Perform information gathering and exploration Learn from past events Be autonomous Requires learning Start with built-in reflexes/knowledge, create new behaviour based on learnt experience Rational agents

10. An agent perceives its environment via sensors and acts in that environment with its effectors. Hence, an agent gets percepts one at a time, and maps this percept sequence to actions (one action at a time) Properties: Autonomous Interacts with other agents plus the environment Reactive to the environment Pro-active (goal-directed) How to design an intelligent agent?

11. sensors percepts ? environment agent actions effectors An agentandits environment

12. Agent type Percepts Actions Goals Environment Medical diagnosis system Symptoms, findings, patient's answers Questions, tests, treatments Healthy patients, minimize costs Patient, hospital Satellite image analysis system Pixels of varying intensity, color Print a categorization of scene Correct categorization Images from orbiting satellite Part-picking robot Pixels of varying intensity Pick up parts and sort into bins Place parts in correct bins Conveyor belts with parts Refinery controller Temperature, pressure readings Open, close valves; adjust temperature Maximize purity, yield, safety Refinery Interactive English tutor Typed words Print exercises, suggestions, corrections Maximize student's score on test Set of students Examples of agents in different types of applications

13. Characterizing Agents

14. Agent’s strategy is a mapping from percept sequence to action How to encode an agent’s strategy? Long list of what should be done for each possible percept sequence vs. shorter specification (e.g. algorithm) Agent’s strategy

15. Skeleton Agent function SKELETON-AGENT (percept) returns action static: memory, the agent’s memory of the world memory UPDATE-MEMORY(memory,percept) action  CHOOSE-BEST-ACTION(memory) memory  UPDATE-MEMORY(memory, action) returnaction On each invocation, the agent’s memory is updated to reflect the new percept, the best action is chosen, and the fact that the action was taken is also stored in the memory. The memory persists from one invocation to the next.

16. Table-driven agent Simple reflex agent Reflex agent with internal state Agent with explicit goals Utility-based agent Examples of how the agent function can be implemented More sophisticated

17. Table-driven agent function TABLE-DRIVEN-AGENT (percept) returns action static: percepts, a sequence, initially empty table, a table, indexed by percept sequences, initially fully specified append percept to the end of percepts action LOOKUP(percepts, table) return action An agent based on a prespecified lookup table. It keeps track of percept sequence and just looks up the best action • Problems • Huge number of possible percepts (consider an automated taxi with a camera as the sensor) => lookup table would be huge • Takes long time to build the table • Not adaptive to changes in the environment; requires entire table to be updated if changes occur

18. Differs from the lookup table based agent in that the condition (that determines the action) is already higher-level interpretation of the percepts Percepts could be e.g. the pixels on the camera of the automated taxi Simple reflex agent

19. Simple Reflex Agent sensors What the world is like now Environment What action I should do now Condition - action rules effectors function SIMPLE-REFLEX-AGENT(percept) returns action static: rules, a set of condition-action rules state  INTERPRET-INPUT (percept) rule  RULE-MATCH (state,rules) action  RULE-ACTION [rule] returnaction First match. No further matches sought. Only one level of deduction. A simple reflex agent works by finding a rule whose condition matches the current situation (as defined by the percept) and then doing the action associated with that rule.

20. Table lookup of condition-action pairs defining all possible condition-action rules necessary to interact in an environment e.g. if car-in-front-is-breaking then initiate breaking Problems Table is still too big to generate and to store (e.g. taxi) Takes long time to build the table No knowledge of non-perceptual parts of the current state Not adaptive to changes in the environment; requires entire table to be updated if changes occur Looping: Can’t make actions conditional Simple reflex agent…

21. sensors State How the world evolves What the world is like now What my actions do Environment What action I should do now Condition - action rules effectors Reflex Agent with Internal State

22. Reflex Agent with Internal State … function REFLEX-AGENT-WITH-STATE (percept) returns action static: state, a description of the current world state rules, a set of condition-action rules state  UPDATE-STATE (state, percept) rule  RULE-MATCH (state, rules) action  RULE-ACTION [rule] state  UPDATE-STATE (state, action) returnaction A reflex agent with internal state works by finding a rule whose condition matches the current situation (as defined by the percept and the stored internal state) and then doing the action associated with that rule.

23. Encode “internal state of the world to remember the past as contained in earlier percepts Needed because sensors do not usually give the entire state of the world at each input, so perception of the environment is captured over time. Requires ability to represent change in the world with/without the agent Reflex Agent with Internal State …

24. sensors State How the world evolves What the world is like now What my actions do Environment What it will be like if I do action A What action I should do now Goals effectors Agent with Explicit Goals

25. Choose actions so as to achieve a (given or computed) goal = a description of desirable situations. e.g. where the taxi wants to go Keeping track of the current state is often not enough – need to add goals to decide which situations are good Deliberative instead of reactive May have to consider long sequences of possible actions before deciding if goal is achieved – involves considerations of the future, “what will happen if I do…?” (search and planning) More flexible than reflex agent. (e.g. rain / new destination) In the reflex agent, the entire database of rules would have to be rewritten Agent with Explicit Goals

26. sensors State How the world evolves What the world is like now What my actions do What it will be like if I do action A Environment How happy I will be in such as a state Utility What action I should do now effectors Utility-Based Agent

27. When there are multiple possible alternatives, how to decide which one is best? A goal specifies a crude destination from an unhappy to a happy state, but often need a more general performance measure that describes “degree of happiness” Utility function U: State  Reals indicating a measure of success or happiness when at a given state Allows decisions comparing choice between conflicting goals choice between likelihood of success and importance of goal (if achievement is uncertain) Utility-Based Agent

28. Characterizing Environments

29. An accessible environment is one in which the agent can obtain complete, accurate, up-to-date information about the environment’s state Most moderately complex environments (including, for example, the everyday physical world and the Internet) are inaccessible The more accessible an environment is, the simpler it is to build agents to operate in it Environments – Accessible vs. Inaccessible

30. A deterministic environment is one in which any action has a single guaranteed effect — there is no uncertainty about the state that will result from performing an action Subjective non-determinism Limited memory Too complex environment to model directly (weather, dice) Inaccessibility The physical world can to all intents and purposes be regarded as non-deterministic Non-deterministic environments present greater problems for the agent designer Environments –Deterministic vs. Non-deterministic

31. The agent’s experience is divided into independent “episodes,” each episode consisting of agent perceiving and then acting. Quality of action depends just on the episode itself, because subsequent episodes do not depend on what actions occur in previous episodes.  Do not need to think ahead Episodic environments are simpler from the agent developer’s perspective because the agent can decide what action to perform based only on the current episode — it need not reason about the interactions between this and future episodes Environments - Episodic vs. Non-episodic

32. A static environment is one that can be assumed to remain unchanged except by the performance of actions by the agent A dynamic environment is one that has other processes operating on it, and which therefore changes in ways beyond the agent’s control Other processes can interfere with the agent’s actions (as in concurrent systems theory) The physical world is a highly dynamic environment Environments - Static vs. Dynamic

33. An environment is discrete if there are a fixed, finite number of actions and percepts in it A chess game is an example of a discrete environment, and taxi driving as an example of a continuous one Continuous environments have a certain level of mismatch with computer systems Discrete environments could in principle be handled by a kind of “lookup table” Environments – Discrete vs. Continuous

34. Environment Accessible Deterministic Episodic Static Discrete Chess with a clock Yes Yes No Semi Yes Chess without a clock Yes Yes No Yes Yes Poker No No No Yes Yes Backgammon Yes No No Yes Yes Taxi driving No No No No No Medical diagnosis system No No No No No Image-analysis system Yes Yes Yes Semi No Part-picking robot No No Yes No No Refinery controller No No No No No Interactive English tutor No No No No Yes

35. Are agents just objects by another name? Object: encapsulates some state communicates via message passing has methods, corresponding to operations that may be performed on this state Agents and Objects

36. Main differences: agents are autonomous: agents embody stronger notion of autonomy than objects, and in particular, they decide for themselves whether or not to perform an action on request from another agent agents are smart: capable of flexible (reactive, pro-active, social) behavior, and the standard object model has nothing to say about such types of behavior agents are active: a multi-agent system is inherently multi-threaded, in that each agent is assumed to have at least one thread of active control Agents and Objects

37. When explaining human activity, it is often useful to make statements such as the following Janine took her umbrella because she believed it was going to rain. Michael worked hard because he wanted to possess a PhD. These statements make use of a folk psychology, by which human behavior is predicted and explained through the attribution of attitudes, such as believing and wanting (as in the above examples), hoping, fearing, and so on The attitudes employed in such folk psychological descriptions are called the intentional notions Agents as Intentional Systems

38. The philosopher Daniel Dennett coined the term intentional system to describe entities ‘whose behavior can be predicted by the method of attributing belief, desires and rational acumen’ Dennett identifies different ‘grades’ of intentional system: ‘A first-order intentional system has beliefs and desires (etc.) but no beliefs and desires about beliefs and desires. …A second-order intentional system is more sophisticated; it has beliefs and desires (and no doubt other intentional states) about beliefs and desires (and other intentional states) — both those of others and its own’ Agents as Intentional Systems

39. Is it legitimate or useful to attribute beliefs, desires, and so on, to computer systems? Agents as Intentional Systems

40. McCarthy argued that there are occasions when the intentional stance is appropriate: Agents as Intentional Systems ‘To ascribe beliefs, free will, intentions, consciousness, abilities, or wants to a machine is legitimate when such an ascription expresses the same information about the machine that it expresses about a person. It is useful when the ascription helps us understand the structure of the machine, its past or future behavior, or how to repair or improve it. It is perhaps never logically required even for humans, but expressing reasonably briefly what is actually known about the state of the machine in a particular situation may require mental qualities or qualities isomorphic to them. Theories of belief, knowledge and wanting can be constructed for machines in a simpler setting than for humans, and later applied to humans. Ascription of mental qualities is most straightforward for machines of known structure such as thermostats and computer operating systems, but is most useful when applied to entities whose structure is incompletely known’.

41. What objects can be described by the intentional stance? As it turns out, more or less anything can. . . consider a light switch: But most adults would find such a description absurd!Why is this? Agents as Intentional Systems ‘It is perfectly coherent to treat a light switch as a (very cooperative) agent with the capability of transmitting current at will, who invariably transmits current when it believes that we want it transmitted and not otherwise; flicking the switch is simply our way of communicating our desires’. (Yoav Shoham)

42. The answer seems to be that while the intentional stance description is consistent, . . . it does not buy us anything, since we essentially understand the mechanism sufficiently to have a simpler, mechanistic description of its behavior. (Yoav Shoham) Put crudely, the more we know about a system, the less we need to rely on animistic, intentional explanations of its behavior But with very complex systems, a mechanistic, explanation of its behavior may not be practicable As computer systems become ever more complex, we need more powerful abstractions and metaphors to explain their operation — low level explanations become impractical. The intentional stance is such an abstraction Agents as Intentional Systems

43. The intentional notions are thus abstraction tools, which provide us with a convenient and familiar way of describing, explaining, and predicting the behavior of complex systems Remember: most important developments in computing are based on new abstractions: procedural abstraction abstract data types objects Agents, and agents as intentional systems, represent a further, and increasingly powerful abstraction So agent theorists start from the (strong) view of agents as intentional systems: one whose simplest consistent description requires the intentional stance Agents as Intentional Systems

44. This intentional stance is an abstraction tool — a convenient way of talking about complex systems, which allows us to predict and explain their behavior without having to understand how the mechanism actually works Much of computer science is concerned with looking for abstraction mechanisms (witness procedural abstraction, ADTs, objects,…)So why not use the intentional stance as an abstraction tool in computing — to explain, understand, and, crucially, program computer systems? This is an important argument in favor of agents Agents as Intentional Systems

45. 3 Other points in favor of this idea: Characterizing Agents: It provides us with a familiar, non-technical way of understanding & explaining agents Nested Representations: It gives us the potential to specify systems that include representations of other systems It is widely accepted that such nested representations are essential for agents that must cooperate with other agents Agents as Intentional Systems

46. Post-Declarative Systems: This view of agents leads to a kind of post-declarative programming: In procedural programming, we say exactly what a system should do In declarative programming, we state something that we want to achieve, give the system general info about the relationships between objects, and let a built-in control mechanism (e.g., goal-directed theorem proving) figure out what to do With agents, we give a very abstract specification of the system, and let the control mechanism figure out what to do, knowing that it will act in accordance with some built-in theory of agency (e.g., the well-known Cohen-Levesque model of intention) Agents as Intentional Systems

47. A multiagent system is one that consists of a number of agents, which interact with one-another In the most general case, agents will be acting on behalf of users with different goals and motivations To successfully interact, they will require the ability to cooperate, coordinate, and negotiate with each other, much as people do Multiagent Systems

48. There are two basic questions we ask when discussing agents How do we build agents capable of independent, autonomous action, so that they can successfully carry out tasks we delegate to them? How do we build agents that are capable of interacting (cooperating, coordinating, negotiating) with other agents in order to successfully carry out those delegated tasks, especially when the other agents cannot be assumed to share the same interests/goals? The first problem is agent design, the second is society design (micro/macro) Agent Design vs Society Design

49. In Multiagent Systems, we address questions such as: How can cooperation emerge in societies of self-interested agents? What kinds of languages can agents use to communicate? How can self-interested agents recognize conflict, and how can they (nevertheless) reach agreement? How can autonomous agents coordinate their activities so as to cooperatively achieve goals? Multiagent Systems

50. While these questions are all addressed in part by other disciplines (notably economics and social sciences), what makes the multiagent systems field unique is that it emphasizes that the agents in question are computational, information processing entities. Multiagent Systems