Again, however, to group books as being of the same kind is not enough; to follow this first rule of reading you must know what that kind is. The title will not tell you, nor all the rest of the front matter, nor even the whole book itself sometimes, unless you have some categories you can apply to classify books intelligently. In other words, this rule has to be made a little more intelligible if you are to follow it intelligently. It can only be made intelligible by drawing distinctions and thus creating categories that make sense and will stand up to the test of time.

We have already discussed a rough classification of books. The main distinction, we said, was between works of fiction, on the one hand, and works conveying knowledge, or expository works, on the other hand. Among expository works, we can further distinguish history from philosophy, and both from science and mathematics.

Now this is all very well as far as it goes. This is a classification scheme with fairly perspicuous categories, and most people could probably place most books in the right category if they thought about it. But not all books in all categories.

The trouble is that as yet we have no principles of classification. We will have more to say about these principles as we proceed in our discussion of the higher levels of reading. For the moment, we want to confine ourselves to one basic distinction, a distinction that applies across the board to all expository works. It is the distinction between theoretical and practical works.