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How do we build systems with “common-sense physics”?

This article explores different methods for supporting plants, including using string, elastic bands, chains, cobwebs, twist ends, large rocks, and shorter stakes. It discusses the concept of "common-sense physics" in building systems and presents two approaches: Naive Physics and Microworlds. It also discusses the challenges and dangers of using microworlds and suggests possibilities for future developments.

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How do we build systems with “common-sense physics”?

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  1. What if... • Stake not stuck in ground? • String only around the plant? • Plant growing in water? • Elastic band? Chain? Cobweb? • Twist ends rather than knot? • Large rock instead of stake? • Stake much shorter than plant? • String much longer? • Stake made out of string? • Stake a tree? The Naive Physics Perplex“Common sense is a wild thing, savage, and beyond rules.”(G. K. Chesterton, 1906) How do we build systems with “common-sense physics”? • Issue  special-purpose solution methods. • Issue = versatility!

  2. The Naïve Physics Perplex by Ernest Davis AI Magazine 19 (4), Winter 1998, pp51-79 As (mis)interpreted by Peter Clark

  3. Common-sense reasoning Typical AI programs “are doing inference in a single direction with complete information or a narrow range of partial information, whereas a general reasoner should do reasoning in many different directions using whatever partial information it has.”

  4. Approach I: “Naive Physics” • Goal:build a single, monolithic KB • “a coherent universal theory” (p67) • Approach: • break it up into clusters (“a nexus of concepts tightly related by a rich collection of axioms”) • Don’t worry about implementation details • Focus: Not just competency, also the theory itself. • content = “what naive subjects believe about the physical world” = ??

  5. Approach II: Microworlds • Goal: Set of small, self-contained theories. • Approach: Model simple, idealized worlds. • Focus: the idealized world itself (“model”), rather than how we represent it. • Example: Mutable Objects (“cutting theory”) World contains: objects, regions, situations. Objects: are material/ghost, are at a place, have a shape. Regions: can be connected. and that is all!! AXIOM: If a blade cuts object X, then X becomes a “ghost” and two new objects Y and Z appear which are “material”.

  6. Microworlds (vs. Naive Physics) • (By definition) ignore issue of “what is naive?” • eg. Is the paint on the wall or partof the wall? • (By definition) can cope with incoherence: • Specific theory for specific task.

  7. Microworlds: Dangers and Difficulties • Not a Task Domain (What is the competence theory of?) • “Use a rolling pin to roll out dough. Then cut it into pieces.” • Why does “it” = “rolling pin”? • Can’t cut rolling pins easily? • It is unusual to cut rolling pins? • Has no purpose in the recipe? • No sources for defining “common-sense inferences” • Innumerable microworlds • Extensional focusomit nebulous concepts eg “purpose” • Excessive mathematization • Too much stress on deduction

  8. Extending & Combining Microworlds “All too often, one finds that each microworld depends on assumptions that are violated in the other.” p71 “Though logic is compositional [new true facts don’t change old true facts], idealization is not. What has struck me in the years that I’ve been doing this is how often extensibility fails; how often adding just a little more realism forces me to go back and reconceptualize the whole thing from the start. Not that the previous work is entirely wasted; generally many of the representational primitives and the axioms survive with little or no change. But it’s rarely straightforward.” (personal comm) “Even in software design, my sense is that ‘upward compatible’ (the analogue of ‘extensible’ or ‘elaboration tolerant’) is more the name of a prayer than the name of an identifiable feature.” (personal comm)

  9. Example: Extension AXIOM: “two objects cannot overlap” O1  O2   intersect(place(O1),place(O2)) [1] Now we move to cutting, where objects might be “ghosts”: REVISED AXIOM: “two material (real) objects cannot overlap” material(O1)  material(O2)  O1  O2   intersect(place(O1),place(O2)) [2] Problem: If we have [1] and want to allow creation/destruction, need to rewrite everything!

  10. Example: Composition • Cutting Theory: • allows chunks to be taken out of things • String Theory: • assumes string has constant cross-section • Cutting String Theory = Cutting  String? • inconsistent assumptions! Fixes: • restrict “chunk” to be “string segment” • allow varying cross-section; but then string theory needs rewriting! • hair-splitting to solve the problem

  11. Possibilities for 2010 (ish)... • Grand Unified KB complete. • Microworlds were just “training exercises” • Library of independent microworlds + metalevel control • Grand Unified KB + Microworlds as efficient approximations

  12. Are Theories Really so Uncomposible? • “Theorem-increasing” extensions: Don’t have to rewrite your existing axioms! • Theory about containers and volumes • Add in portals (to move stuff in/out) • Add in portal may have a door… • Add in door may be lockable...

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