In A Study in Scarlet, Arthur Conan Doyle has John Watson describe one of his earliest conversations with Sherlock Holmes:
His ignorance was as remarkable as his knowledge. Of contemporary literature, philosophy and politics he appeared to know next to nothing. Upon my quoting Thomas Carlyle, he inquired in the naivest way who he might be and what he had done. My surprise reached a climax, however, when I found incidentally that he was ignorant of the Copernican Theory and of the composition of the Solar System. That any civilized human being in this nineteenth century should not be aware that the earth travelled round the sun appeared to me to be such an extraordinary fact that I could hardly realize it.
“You appear to be astonished,” he said, smiling at my expression of surprise. “Now that I do know it I shall do my best to forget it.”
“To forget it!”
“You see,” he explained, “I consider that a man’s brain originally is like a little empty attic, and you have to stock it with such furniture as you choose. A fool takes in all the lumber of every sort that he comes across, so that the knowledge which might be useful to him gets crowded out, or at best is jumbled up with a lot of other things, so that he has a difficulty in laying his hands upon it. Now the skillful workman is very careful indeed as to what he takes into his brain-attic. He will have nothing but the tools which may help him in doing his work, but of these he has a large assortment, and all in the most perfect order. It is a mistake to think that that little room has elastic walls and can distend to any extent. Depend upon it there comes a time when for every addition of knowledge you forget something that you knew before. It is of the highest importance, therefore, not to have useless facts elbowing out the useful ones.”
“But the Solar System!” I protested.
“What the deuce is it to me?” he interrupted impatiently: “you say that we go round the sun. If we went round the moon it would not make a pennyworth of difference to me or to my work.”
It would appear that Holmes was on to something. Efficient brains need to know what they can afford to forget—probably quite a bit, now that it’s so easy to outsource our recollections to rapidly-searched digital media. The interesting question for me is: When almost anything you might need to recall can be offloaded in this way, what’s worth keeping in wetware memory? My first instinct is that you need to remember exactly enough to (1) make interesting connections, and (2) actually find the full information from the signpost you’ve remembered. I certainly didn’t remember the verbatim passage quoted above, nor that it was from A Study in Scarlet—though I probably could’ve figured that much out if I’d thought about it for a minute, given Watson’s unfamiliarity with Holmes’ methods.
Of course, a lot of this depends on the medium. If you’re doing general-interest writing, you probably want to skew broad-and-shallow, given that you have the time to look things up. If you’re having a live debate or engaged in witty repartee in realtime, you want to go narrower and deeper, have just the right line from “Dover Beach” or King Lear or the decision in Board v. Barnette at your fingertips. This may entail that the practice of having successful writers on TV or radio as commentators—even leaving aside the familiar disconnect between skill in written prose and smooth oral presentation—may make less and less sense in an age where everything can be looked up. The optimal scribbler will know everything in vague outline, nothing in detail.