What do we know about knowledge?

Intelligent systems need 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.