What Is A Knowledge Representation? 6.871 - Lecture 13Outline • What Is A Representation? –Five Roles • What Should A Representation Be? • What Consequences Does This View Have For Research And Practice? – One answer to a foundational question – The “spirit” of a representation • The spirit should be indulged – In analysis – In system construction – The central task of knowledge representation 6.871 - Lecture 13 2Terminology and Perspective • Inference = getting new expressions from old Not limited to deductive (sound) inference. • “Knowledge Representation Technologies”: rules, frames, logic, semantic nets, etc. 6.871 - Lecture 13 3What Is a KR? 1) It’s a surrogate. A substitute for the thing itself. 2) It’s a set of ontological commitments. In what terms should I think about the world? 3) It’s a fragment of a theory of intelligent reasoning. – What is intelligence? – What can I infer from what I know? (i.e., which inferences are sanctioned?) – What should I infer from what I know? (i.e., which inferences are recommended?) 6.871 - Lecture 13 4What Is a KR? 4) It’s a medium for pragmatically efficient computation. The computational medium in which thinking is accomplished. How should I organize information to facilitate that thinking? 5) It’s a medium of expression and communication. A language we use to talk to the machine. 6.871 - Lecture 13 5[1] It’s A Surrogate• A stand-in for the object in the real world. • Operations on the KR substitute for actions in the world. • Reasoning is itself a substitute for action. • (Conversely, actions can substitute for reasoning). 6.871 - Lecture 13 6[1] It’s A Surrogate• Questions: – A surrogate for what? – How accurate a surrogate? 6.871 - Lecture 13 7[1] It’s A Surrogate• Questions: – A surrogate for what? Æ semantics – How accurate a surrogate? 6.871 - Lecture 13 8[1] It’s A Surrogate• Questions: – A surrogate for what? Æ semantics – How accurate a surrogate? Æ fidelity 6.871 - Lecture 13 9[1] It’s A Surrogate• Questions: – A surrogate for what? Æ semantics – How accurate a surrogate? Æ fidelity • More fidelity is not automatically better • Perfect fidelity is impossible. 6.871 - Lecture 13 10[1] It’s A SurrogatePerfect fidelity is impossible Æ We inevitably lie. Æ Incorrect inferences are inevitable. Sound reasoning can’t save us. A better representation can’t save us. Æ We have already sinned. Æ We may as well be pragmatic about it. 6.871 - Lecture 13 11[2] Set of Ontological Commitments • Surrogates are inevitably imperfect Æ KR selection unavoidably makes an OC. • Commitment occurs even at the level of the KRT’s – Diagnosis as rules vs. frames. 6.871 - Lecture 13 12[2] Set of Ontological Commitments • The commitment accumulates in layers – EG: medical diagnosis •frames Æ prototypes, defaults, taxonomy • prototypes of what? • what diseases? 6.871 - Lecture 13 136.871 - Lecture 13 14[2] Set of Ontological CommitmentsAlcoholism as disease: View comes into questionSaying that standard treatments have not helped many drugs and alcohol abusers, some people on the front lines of treatment are challenging the view of addiction as a disease controlled by biological factors.The idea that alcoholism is a disease that can be treated like other chronic medical conditions has been doctrine since the 1960s. By the ’80s, the biomedical model had become so dominant that many researchers believed it was only a matter of time before a gene for alcoholism would be found.Now, however, some specialists are backing off from that stand. While biology certainly plays a role in addiction, they say, it isn’t the whole or even most of the story. The social and environmental factors behind why people drink or take drugs, as well as their beliefs about how their drug of choice helps them, are just as crucial in understanding and treating addictions, they argue.“The negative effects of stressful life events, socioeconomic status and unemployment are key factors,” says Emil Chiauzzi, clinical director of addiction services for the Deaconess-Waltham Hospital. “And people do not become addicted only to a drug, but to the drug experience, the rituals involved.”4/9/06 p.1 Adapted fromBoston Globe[2] Set of Ontological Commitments• The commitment accumulates in layers – EG: medical diagnosis •frames Æ prototypes, defaults, taxonomy • prototypes of what? • what diseases? • Commitment is inevitable • Commitment is crucial 6.871 - Lecture 13 15[3] Fragment of a Theory of Intelligent Reasoning • What are all the inferences am I permitted to make? – Example: classical formal logic; sound inference – Other answers • Logic: circumscription • Rules: plausible inference • Frames: good matches, expectations, defaults. 6.871 - Lecture 13 16[3] Fragment of a Theory of Intelligent Reasoning • What are all the inferences am I permitted to make? – Logic: sound inference • Which inferences am I encouraged to make? – Example: Frames • What reasoning to do: anticipatory matching – Other examples • SN: propagation; links. • Rules: chaining; associations. • Logic: lemmas; connection graphs. – Combinatorial explosions Æ the need for guidance on what we should do, not only what we can do. 6.871 - Lecture 13 17[4] Medium for pragmatically efficient computation • Reasoning with KR means computing with it. • The pendulum swing – Heuristic adequacy (1969) – The logicist view (circa 1974) – The computational imperative view (circa 1984) 6.871 - Lecture 13 18[4] Medium for pragmatically efficient computation • The pragmatics of it: How can I organize information to facilitate reasoning? – Example: Frames — triggers, procedural attachment, taxonomic hierarchies 6.871 - Lecture 13 19[5] Medium of Expression and Communication • It’s how we say things about the world. • It’s how we communicate with the reasoner. • In principle, as a medium of expression: – How general, how precise? – Does it provide expressive adequacy? • In practice, as a medium of communication: – How transparent is it? Can we understand what’s been said? Can we generate the right expression? 6.871 - Lecture 13 20What Should A Representation Be? 1) Surrogate: imperfect fidelity Æ incorrect inference is inevitable Æ take a pragmatic