单项选择题

TEXT A
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.
"You appear to be astonished," Holmes said, smiling at my expression. "Now that I do know it I shall do my best to forget it. You see, 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 jumbled up with a lot of other things, so that he has difficulty in laying his hand upon it. It is a mistake to think that the 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 know 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.
One morning, I picked up a magazine from the table and attempted to while away the time with it, while my companion munched silently at his toast. One of the articles had a pencil mark at the heading, and I naturally began to run my eye through it.
Its somewhat ambitious title was "The Book of Life," and it attempted to show how much an observant man might learn by an accurate and systematic examination of all that came in his way. It struck me as being a remarkable mixture of shrewdness and of absurdity. The reasoning was close and intense, but the deduction appeared to me to be far-fetched and exaggerated. The writer claimed by a momentary expression, a twitch of a muscle or a glance of an eye, to fathom a man’s inmost thought. Deceit, according to him, was impossibility in the case of one trained to observation and analysis. His conclusions were as infallible as so many propositions of Euclid. So startling would his results appear to the uninitiated that until they learned the processes by which he had arrived at them they might well consider him as a necromancer.
"From a drop of water, "said the writer, "a logician could infer the possibility of an Atlantic. So all life is a great chain, the nature of which is known whenever we are shown a single link of it. Like all other arts, the science of Deduction and Analysis is one which can be acquired by long and patient study, nor is life long enough to allow any mortal to attain the highest possible perfection in it."
This smartly written piece of theory I could not accept until a succession of evidences justified it.
What was the Holmes’ idea about knowledge-learning