The author's main purpose in this book is to teach precision in writing; and of good writing (which, essentially, is clear thinking made visible) precision is the point of capital concern. It is attained by choice of the word that accurately and adequately expresses what the writer has in mind, and by exclusion of that which either denotes or connotes something else. As Quintilian puts it, the writer should so write that his reader not only may, but must, understand.