Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!watmath!clyde!cbatt!cbosgd!ihnp4!inuxc!pur-ee!uiucdcs!uiucme!keith From: keith@uiucme.UUCP Newsgroups: net.cog-eng Subject: knowledge and design Message-ID: <11800010@uiucme> Date: Thu, 3-Jul-86 12:50:00 EDT Article-I.D.: uiucme.11800010 Posted: Thu Jul 3 12:50:00 1986 Date-Received: Thu, 10-Jul-86 04:48:52 EDT Lines: 113 Nf-ID: #N:uiucme:11800010:000:6221 Nf-From: uiucme.UUCP!keith Jul 3 11:50:00 1986 What Sussman Said - and What He Didn't eleventh of a series One of the fine old traditions of the middle ages was the commentary. One chose a publication of general interest and rewrote it, adding comments where suitable. Many of the Greek and Latin authors from B.C. are preserved only in this format - their original writings were destroyed when soldiers cleaned up the infidel's library's. I would like to comment on a paper by G.J. Sussman of M.I.T. The paper was published in the late '70s, introducing the notion of solving a problem by propogation of constraints (a concept implemented, if incompletely, in spreadsheet programs). It is available in two, nearly identical versions: Sussman, G.J., SLICES, At the Boundary Between Analysis and Synthesis, MIT AI Lab Memo 433, July 13, 1977. Sussman, G.J., SLICES, At the Boundary Between Analysis and Synthesis, in Latombe, J. (Editor), ARTIFICIAL INTELLIGENCE AND PATTERN RECOGNITION IN COMPUTER-AIDED DESIGN, North-Holland, 1978. (Proceedings af an IFIP working group conference). My aim here is to add some supplemental comments to an existing work of great value. Please don't misconstrue my comments as criticism - not having a doctoral degree myself I am not permitted to criticise those who do. We feel that analytic techniques are a necessary component of synthetic reasoning. Why is this? The processes of synthesis (moving information down the plan into greater detail) and analysis (moving information up the plan) seem exactly opposite. This seems to be the root of not a dissociation but an association. By understanding analysis methods, one can reason in reverse in order to synthesize. This does not mean that one can simply reverse the analysis procedure and follow a procedure for synthesis. (One cannot reverse an algorithm - what is the meaning of arriving at a decision through the "false" branch?) Rather, one must invoke a problem-solving skill and exercise some judgement in determining the steps to be taken. To some extent this is intuitive, and those who are adept at make correct intuitive decisions are sometimes called creative. By better understanding not only analysis (domain knowledge) but also problem- solving (a skill) we can be better at synthesis. Analysis methods are not always quantitative. Qualitative reasoning, often dismissed as "hand-waving", is essential to design. Sussman writes "Believing in the ultimate power of mathematical manipulation is one of the most common difficulties encountered by students learning electrical circuit [or any other kind of] analysis. Students often grind out 'impossible' algebra in the course of solving a homework problem, even though a little thought will reveal an algebraically feasible approach which depends upon a small insight into the operation of the network being analyzed. They then complain that we give them too much homework!" This small insight is often not expressed explicitly in lecture or textbook (in fact it may not be expressible). While I hold strong personal feelings against difficult homework assignments, I feel compelled to point out that we learn most readily from our mistakes, and that grinding out the impossible solution may be the most effective method for encouraging the student to seek and recognize the feasbile method next time. Problem-solving is a skill, not a knowledge, and skills are developed and maintained through vigorous exercise. We observe that a common and vitally important kind of small insight is knowing the form of the answer. If we can recognize the general shape the solution will have, then it is possible to add detail to that abstract solution by filling gaps until the design is complete (Sussman calls this "problem solving by debugging almost-right plans"). There are methods which we are beginning to understand for developing a picture of the solution (more to be said in a later instalment). In simple problems the form of the answer proposes itself directly from the form of the problem, in more complex problems it is necessary to manipulate the problem - decomposing or filling missing details - before the form of the solution will be perceivable. An example is discussed in detail, a dummy load for an antenna which must dissipate 500KW through 50 ohms. The notion of "slices" is introduced, which are parallel (in the sense of reasoning, not in the electric circuit sense) versions of the same circuit - for example Thevenin and Norton equivalents. These slices each describe an important attribute of the system and its functions. At this point the idea of knowing the form of the answer is not applied as well as it might. In an intuitive leap, the dummy load becomes an RLC network and the analysis seems to concentrate on obtaining appropriate component values. The synthesis problem is perceiving the topology of such a circuit - how the components might relate in order to achieve such a goal - in other words the form of the solution. The bit of knowledge needed is Ohm's law - not the calculation but the observation that two resistors in parallel can team up to achieve a greater amount or resistance than their sum. Added to that is the knowledge that impedances act like resistances in this context, and the frequency-impedance relationships of inductors and capacitors. None of this involves calculation, rather it involves seeing a chain or relationships that can achieve the goal. To better understand your own problem-solving, look back over a solution and ask yourself what basic knowledge was necessary and how the bits connected together. The key point in the paper, to me, was the notion of knowing the form of the answer. This was not the main thrust of the paper and no doubt my detailed interest in one aspect explains my ability to add detail to one segment of the paper. Armed with the knowledge that the first step is to obtain a form, rather than a complete description, one can on that basis alone work more effectively as a designer. keith U of Illinois Mech Eng seismo!ihnp4!uiucdcs!uiucme!keith next: Naive Physics and competent design (in which it is reveals how limited my knowledge of a.i. really is)