AI Magazine Volume 26 Number 4 (2006)(2005) (© AAAI) Articles IAAI/AI Magazine 2006 Robert Engelmore Award Address What Do We Know about Knowledge? Bruce G. Buchanan ■ Intelligent systems need knowledge. However, the stand nature,4 but the concept of knowledge is simple equation “knowledge is power” leaves three much the same. major questions unanswered. First, what do we Bacon was among the first of the modern mean by “knowledge”; second, what do we mean philosophers to separate the concept of scien- by “power”; and third, what do we mean by “is”? tific knowledge from knowledge gained In this article, I will examine the first of these ques- through the two dominant methods for attain- tions. In particular I will focus on some of the mile- stones in understanding the nature of knowledge ing truth in his time: magic and religious reve- and some of what we have learned from 50 years lation. The essential difference for him, as for of AI research. The discipline and detail required to us, is that knowledge gained through experi- write programs that use knowledge have given us ment is replicable by others. some valuable lessons for implementing the Although all the empirical sciences rely on knowledge principle, one of which is to make our the replication of observations and experi- programs as flexible as we can. ments, AI has been slow to embrace this prin- ciple.5 Programs demonstrating research ideas in AI are often too large and not well enough documented to allow replication or sharing. Applications programs, however, are designed hank you for this distinguished award to be used by others outside the research lab and the opportunity to share some and thus are more amenable to multiple runs Tthoughts with you.1 What I would like to in diverse conditions. Thus they have the give you in this article2 are some of the princi- potential to provide experimental data demon- ples guiding the implementation of knowl- strating strengths, weaknesses, and benefits. edge-based systems that follow from work in philosophy and AI. Many of them are well Contributions before AI known, but they can serve as reminders of the difficulty of implementing the “knowledge is The knowledge principle predates Bacon. For power” principle.3 I wish to clarify the knowl- example, it was pretty clearly articulated in Bib- edge principle and try to increase our under- lical times: “A man of knowledge increaseth standing of what programmers and program strength” (Proverbs 24: 5). designers need to do to make the knowledge Socrates, Plato, Aristotle, and other early principle work in practice. Greek philosophers based their lives on acquir- The “knowledge is power” principle is most ing and transferring knowledge. In the course closely associated with Francis Bacon, from his of teaching, they sought to understand the 1597 tract on heresies: “Nam et ipsa scientia nature of knowledge and how we can establish potestas est.” (“In and of itself, knowledge is knowledge of the natural world. power.”) Incidentally, Bacon was probably as Socrates is famous for pointing out the value much interested in the political power to be of knowledge and seeking truth, as in “… that gained from knowledge as the power to under- which we desire to have, and to impart to oth- Copyright © 2006, American Association for Artificial Intelligence. All rights reserved. ISSN 0738-4602 WINTER 2006 35 Articles ers, [is] expert knowledge….” (Plato, Phaedrus son. Aristotle wrote at least 31 treatises describ- 270d). ing every aspect of the natural world and offer- He was also fond of pointing out how little ing physical explanations of many phenomena. we actually know—and was put to death, Aristotle also advanced the rational tradition essentially, for pointing that out to everyone: of Plato and Pythagoras by developing a logic When I conversed with him I came to see that, that captures many forms of symbolic argu- though a great many persons, and most of all ment, which was powerful enough to survive he himself, thought that he was wise, yet he two thousand years. He demonstrated the was not wise. Then I tried to prove to him that expressive power of simple propositions, “A is he was not wise, though he fancied that he was. B,” along with quantification, “All A’s are B’s,” … I thought to myself, “I am wiser than this or “Some A’s are B’s.” He also established rules man: neither of us knows anything that is really of symbolic inference for combining quanti- worth knowing, but he thinks that he has fied propositions. knowledge when he has not, while I, having no knowledge, do not think that I have. I seem, at Euclid’s geometry firmly established the con- any rate, to be a little wiser than he is on this cept of rigorous proof within mathematics. point: I do not think that I know what I do not Some of the Greek philosophers’ contributions know.” Next I went to another man… (Plato, to our concept of knowledge are highlighted in Apology VI:22). table 1. Plato, Socrates’s pupil and Aristotle’s mentor, In the intervening several centuries before was the first to pose the question in writing of the Middle Ages and the rise of modern science what we mean when we say that a person in the West,6 the search for knowledge was knows something about the world (Cornford overwhelmed by the power of the Christian 1935). He was distinguishing empirical knowl- church to make new knowledge fit with estab- edge, lacking complete certainty, from the cer- lished dogma. The resulting dark ages should tain knowledge of mathematics. The whole be a reminder to all of us that knowledge-based dialogue, The Theaetetus, is worth reading but— systems should not merely perpetuate the if I may oversimplify the conclusion—Plato, established dogma of an organization. speaking for Socrates, concludes that person S The English theologian and philosopher knows empirical proposition p if and only if: Robert Grosseteste (1170–1253) is known for S believes p emphasizing the role of mathematics in under- p is true (otherwise it is a false belief, not a fact standing the natural world. Galileo later under- that is known) scored this principle when he wrote that the S can provide a rationale for why p is true “book of nature” is written in “the language of (which Plato calls giving an account). mathematics.” Grosseteste is also credited with The last condition has been modified by establishing the experimental method as a philosophers in recent years to read that S is path to knowledge in his own experimental justified in believing p. This modification pre- work on the refraction of light. serves the requirement of a rationale but William of Ockham was the most influential removes the onus of providing it from subject philosopher of the 13th century. His two major S. That is, the belief is justified, but S does not contributions to the study of knowledge that need to be the one providing it. But, of course, are relevant to AI were nominalism and his philosophers are not at all in agreement about insistence on simplicity. With nominalism, he what constitutes a proper justification or argued that what we know is expressed in lan- rationale. One view that would seem to be rel- guage. As a pragmatic principle in AI program- evant for AI is that either the belief or the jus- ming, that translates roughly into the principle tification is formed through a reliable cognitive that if someone can accurately describe how process. S didn’t just come to believe p through they solve a problem, then a program can be a series of bad inferences or guessing based on written to solve it. The principle of parsimony, the wrong reasons (Steup 2006). now known as Occam’s Razor, states that plu- Aristotle continued the search for knowl- rality should not be assumed without necessity. In edge, extending the methodology in two other words, explanations that mention fewer important ways beyond the rational discussion entities and mechanisms should be preferred to of Plato and the mathematics of Pythagoras. His more complex ones. term for science, incidentally, was “natural phi- So, by the time modern science was getting losophy,” which was used by scientists as late as started, several important principles about Newton to describe their own work. One of knowledge had already been clearly established Aristotle’s most lasting contributions was show- by the ancient Greeks. Medieval philosophers ing the importance of knowledge gained reinforced and added to the early concepts, as through observation, as opposed to pure rea- shown in table 2. 36 AI MAGAZINE Articles Pythagoras Mathematics holds the key to correct descriptions of theworld. Socrates Seeking knowledge is good. Knowing what we don’t know (metaknowledge) isvaluable. Socrates Empiricalknowledge istruebelief with an account: beliefs have to be justified to be called (Plato) knowledge. Aristotle Observation isalegitimate source of knowledge. Symbolic logic isameansofincreasing our storeofknowledge through valid inference: Knowledge beyond mathematics can be proved. Euclid New knowledge can be derived by rigorousproof. Table 1. Some Contributions of Early Greek Philosophers to Our Understanding of the Concept of Knowledge. R. Grosseteste Mathematics isessential forknowledge of thenatural world. Knowledge can be established experimentally. William of Knowledge is linguistic. Simpler expressions of knowledge are Ockham preferable. Table 2. Some Contributions of Medieval Philosophers to our Understanding of the Concept of Knowledge. Skipping ahead a few more centuries into emphasized planned experiments as an essen- the 15th and 16th centuries, philosophers con- tial step in the inductive process. tinued to investigate scientific questions by René Descartes is most known today for his elaborating the earlier themes and by making work on algebra and geometry, but he also new distinctions.