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What Do We Know about Knowledge? However, the simple equation "knowledge is power" leaves three major questions unanswered. First, what do we mean by "knowledge"; second, what do we mean by "power"; and third, what do we mean by "is"? In this article, I will examine the first of these questions. In particular I will focus on some of the milestones in understanding the nature of knowledge and some of what we have learned from 50 years of AI research. The discipline and detail required to write programs that use knowledge have given us some valuable lessons for implementing the knowledge principle, one of which is to make our programs as flexible as we can. Many of them are well known, but they can serve as reminders of the difficulty of implementing the "knowledge is power" principle. I wish to clarify the knowledge principle and try to increase our understanding of what programmers and program designers need to do to make the knowledge principle work in practice. The "knowledge is power" principle is most closely associated with Francis Bacon, from his 1597 tract on heresies: "Nam et ipsa scientia potestas est." ("In and of itself, knowledge is power.") Bacon was among the first of the modern philosophers to separate the concept of scientific knowledge from knowledge gained through the two dominant methods for attaining truth in his time: magic and religious revelation. The essential difference for him, as for us, is that knowledge gained through experiment is replicable by others. Although all the empirical sciences rely on the replication of observations and experiments, AI has been slow to embrace this principle. 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 to be used by others outside the research lab and thus are more amenable to multiple runs in diverse conditions. Thus they have the potential to provide experimental data demonstrating strengths, weaknesses, and benefits. The knowledge principle predates Bacon. For example, it was pretty clearly articulated in Biblical times: "A man of knowledge increaseth strength" (Proverbs 24: 5). Socrates, Plato, Aristotle, and other early Greek philosophers based their lives on acquiring and transferring knowledge. In the course of teaching, they sought to understand the nature of knowledge and how we can establish knowledge of the natural world. Socrates is famous for pointing out the value of knowledge and seeking truth, as in "… that which we desire to have, and to impart to oth-