If today there is a sharp distinction between astronomy, the science of stars, and astrology, using horoscopes to make prediction, that has not always been the case. Before the seventeenth century, the words astronomy and astrology were often used interchangeably, and most astronomers (for example, Ptolemy and Kepler) were also astrologers. In this talk, we will examine how the distinction between astronomy and astrology had been puzzling scholars for centuries, and how a consensus was finally found in the early modern period. Behind this question of terminology lies the problem of distinguishing good and bad knowledge, and judging the heritage of the past. It is a question about the bases and norms of scientific knowledge. It is a question about building boundaries in the intellectual world.