As the biggest library if it is in disorder is not as useful as a small but well-arranged one, so you may accumulate a vast amount of knowledge but it will be of far less value to you than a much smaller amount if you have not thought it over for yourself; because only through ordering what you know by comparing every truth with every other truth can you take complete possession of your knowledge and get it into your power. You can think about only what you know, so you ought to learn something; on the other hand, you can know only what you have thought about.
Now you can apply yourself voluntarily to reading and learning, but you cannot really apply yourself to thinking; thinking has to be kindled, as a fire is by a draught, and kept going by some kind of interest in its object, which may be an objective interest or merely a subjective one. The latter is possible only with things that affect us personally, the former only to those heads who think by nature, to whom thinking is as natural as breathing, and these are very rare. That is why most scholars do so little of it.