In previous chapters, readers were told that expository books are those which convey knowledge. Within the classification of expository books, there is also a division between practical books (those “concerned with the problems of action”) and theoretical books (those “concerned only with something to be known;” 188). Furthermore, practical books can be divided into those which present rules and those which are “primarily concerned with the principles that generate rules” (190, emphasis added). According to Adler and Van Doren, “in reading a book that is primarily a rule-book, the major propositions to look for, of course, are the rules. A rule is most directly expressed by an imperative rather than a declarative sentence” (190-91). In reading a book dealing with principles, “the major propositions and arguments will, of course, look exactly like those in a purely theoretical book” (191).

Two questions that readers must ask themselves when reading a practical book are “what are the author’s objectives?” and “what means for achieving them is he proposing?” (193). The questions operate under the logic that the author is trying to persuade the reader. Adler and Van Doren argue that “it is the very nature of practical affairs that men have to be persuaded to think and act in a certain way” (193).