In Book II, Quintilian defines rhetoric as an art, while classifying the three types of arts: "theoretical", "practical", and "productive" (2.17-18). He concludes that rhetoric partakes of all three categories, but associates it most strongly with the "practical" (2.18.1-5). Rhetoric is also divided into three categories: (1) art, (2) artist, and (3) work (2.14.5). Quintilian then moves into an exploration of rhetoric's nature and virtue, following it with a comparison of oratory and philosophy (2.19-21). It should also be noted that Quintilian uses these two terms, rhetoric and oratory, interchangeably (see Book II).