Evidently then, even if it is good advice to write what you know, that doesn't have to be anything the you know to be true. In fact, if you're writing fiction you can't write anything really true, although you should know something about the underlying topic of you story. By contrast, for writing a philosophic or a scientific discourse, or when writing in the history of ideas, a knowledge of the "truth" of certain facts would seem to be a prerequisite. For instance, if you're trying to persuade your readers that the trajectory of the space craft launched three years ago will not take the craft close enough to Pluto to get good pictures, you must know what the current trajectory is computed to be. Or if you're trying to convince a colleague that some philosopher's position on some issue is weak because he has contradicted himself in his texts, you must know what the pertinent texts actually say. Concerning such topics, there are just no two ways about it.
But of course, not all topics are like this; and about some things there certainly are two ways, or more. In such cases it behooves the rhetor to have a well-rounded understanding of the topics. In fact, it is probably safe to say that in all cases it behooves the rhetor to have a well-rounded understanding, so that this understanding may be applied differently in different cases, as the need arises. This, of course, is just another way of saying that the rhetor should be flexible in his or her approach. And this flexibility will come, at least in part, from having a working acquaintance with as many different areas of learning and fields of knowledge as is humanly possible. Is this not, indeed, the aim of the liberal education? Most certainly, and given this liberal education—an education in the Arts, the Sciences, and the Humanities—we will be able to draw upon a wide range of topics when choosing an appropriate subject for discussion.
The widest range of topics, it would seem, is the whole of our cultural heritage. And after some ten thousand or so years of civilization, we humans have developed an astounding array of topics from which to choose. And these days we may choose not only from traditional sources, such as religion and myth, but also from more recent sources, as we can see on the next page.