Almost always it pays to give two or three hours to some preliminary reading that will make you see the general scope of the subject, and the points on which there is disagreement. An article in a good encyclopedia or one in a magazine may serve the purpose; or in some cases you can go to the opening chapter or two of a book. If you have already discussed the subject with other people this preliminary reading may not be necessary; but if you start in to read on a new subject without some general idea of its scope you may waste time through not knowing your way and so following false leads.
In your reading do not rest satisfied with consulting authorities on your own side only. We shall presently see how important it is to be prepared to meet arguments on the other side; and unless you have read something on that side, you will not know what points you ought to deal with in your refutation. In that event you may leave undisturbed in the minds of your readers points which have all the more significance from your having ignored them. One of the first reasons for wide reading in preparation for an argument is to assure yourself that you have a competent knowledge of the other side as well as of your own.
In using your sources keep clearly and constantly in mind the difference between fact and opinion. The opinions of a great scholar and of a farseeing statesman may be based on fact; but not being fact they contain some element of inference, which is never as certain. When we come to the next chapter we shall consider this difference more closely. In the meantime it is worth while to urge the importance of cultivating scruples on the subject and a keen eye for the intrusion of human, and therefore fallible, opinion into statements of fact. A trustworthy author states the facts as facts, with the authorities for them specifically cited; and where he builds his own opinions on the facts he leaves no doubt as to where fact ends and opinion begins.
The power to estimate a book or an article on a cursory inspection is of great practical value. The table of contents in a book, and sometimes the index, will give a good idea of its scope; and samples of a few pages at a time, especially on critical points, which can be chosen by means of the index, will show its general attitude and tone. The index, if properly made, will furnish a sure guide to its relevance for the purpose in hand. Half an hour spent in this way, with attention concentrated, will in most cases settle whether the book is worth reading through. An article can be “sized up” in much the same way: if it is at all well written the first paragraphs will give a pretty definite idea of the subject and the scope of the article; and the beginnings, and often the ends, of the paragraphs will show the course which the thought follows. Though such skimming cannot be relied on for a real knowledge of the subject, it is invaluable as a guide for this preliminary reading.