Found via Matt Brice.
Excerpt from C.S. Lewis' introduction to Athanasius: On the Incarnation:
There is a strange idea abroad that in every subject the ancient books should be read only by the professionals, and that the amateur should content himself with the modern books. Thus I have found as a tutor in English Literature that if the average student wants to find out something about Platonism, the very last thing he thinks of doing is to take a translation of Plato off the library shelf and read the Symposium. He would rather read some dreary modern book ten times as long, all about "isms" and influences and only once in twelve pages telling him what Plato actually said. The error is rather an amiable one, for it springs from humility. The student is half afraid to meet one of the great philosophers face to face. He feels himself inadequate and thinks he will not understand him. But if he only knew, the great man, just because of his greatness, is much more intelligible than his modern commentator. The simplest student will be able to understand, if not all, yet a very great deal of what Plato said; but hardly anyone can understand some modern books on Platonism. It has always therefore been one of my main endeavours as a teacher to persuade the young that firsthand knowledge is not only more worth acquiring than secondhand knowledge, but is usually much easier and more delightful to acquire.
Naturally, since I myself am a writer, I do not wish the ordinary reader to read no modern books. But if he must read only the new or only the old, I would advise him to read the old. And I would give him this advice precisely because he is an amateur and therefore much less protected than the expert against the dangers of an exclusive contemporary diet. A new book is still on its trial and the amateur is not in a position to judge it....It is a good rule, after reading a new book, never to allow yourself another new one till you have read an old one in between. If that is too much for you, you should at least read one old one to every three new ones.
Every age has its own outlook. It is specially good at seeing certain truths and specially liable to make certain mistakes. We all, therefore, need the books that will correct the characteristic mistakes of our own period. And that means the old books. All contemporary writers share to some extent the contemporary outlook—even those, like myself, who seem most opposed to it. Nothing strikes me more when I read the controversies of past ages than the fact that both sides were usually assuming without question a good deal which we should now absolutely deny. They thought that they were as completely opposed as two sides could be, but in fact they were all the time secretly united—united with each other and against earlier and later ages—by a great mass of common assumptions. We may be sure that the characteristic blindness of the twentieth century—the blindness about which posterity will ask, "But how could they have thought that?"—lies where we have never suspected it, and concerns something about which there is untroubled agreement between Hitler and President Roosevelt or between Mr. H. G. Wells and Karl Barth. None of us can fully escape this blindness, but we shall certainly increase it, and weaken our guard against it, if we read only modern books. Where they are true they will give us truths which we half knew already. Where they are false they will aggravate the error with which we are already dangerously ill. The only palliative is to keep the clean sea breeze of the centuries blowing through our minds, and this can be done only by reading old books. Not, of course, that there is any magic about the past. People were no cleverer then than they are now; they made as many mistakes as we. But not the same mistakes. They will not flatter us in the errors we are already committing; and their own errors, being now open and palpable, will not endanger us. Two heads are better than one, not because either is infallible, but because they are unlikely to go wrong in the same direction. To be sure, the books of the future would be just as good a corrective as the books of the past, but unfortunately we cannot get at them.