“there comes a time when for every addition of knowledge you forget something that you knew before.”

“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.”

“That any civilized human being in this nineteenth century should not be aware that the earth traveled 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 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 difficulty in laying his hands upon it. Now the skilful 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 most perfect order. It is a mistake to think that 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.”

“The swing of his nature took him from extreme languor to devouring energy; and as I knew well, he was never so truly formidable as when, for days on end, he had been lounging in his armchair amid his improvisations and his black-letter editions. Then it was that the lust of the chase would suddenly come upon him, and that his brilliant reasoning power would rise to the level of intuition, until those who were unacquainted with his methods would look askance at him as on a man whose knowledge was not that of other mortals. When I saw him that afternoon so enwrapped in the music of St. James's Hall I felt that an evil time might be coming upon those whom he had set himself to hunt down.”

“Dr. Watson's summary list of Sherlock Holmes's strengths and weaknesses:"1. Knowledge of Literature: Nil.2. Knowledge of Philosophy: Nil.3. Knowledge of Astronomy: Nil.4. Knowledge of Politics: Feeble.5. Knowledge of Botany: Variable. Well up in belladonna, opium, and poisons generally. Knows nothing of practical gardening.6. Knowledge of Geology: Practical but limited. Tells at a glance different soils from each other. After walks has shown me splashes upon his trousers, and told me by their colour and consistence in what part of London he had received them.7. Knowledge of Chemistry: Profound.8. Knowledge of Anatomy: Accurate but unsystematic.9. Knowledge of Sensational Literature: Immense. He appears to know every detail of every horror perpetrated in the century.10. Plays the violin well.11. Is an expert singlestick player, boxer, and swordsman.12. Has a good practical knowledge of British law.”

“Problems may be solved in the study which have baffled all those who have sought a solution by the aid of their senses. To carry the art, however, to its highest pitch, it is necessary that the reasoner should be able to use all the facts which have come to his knowledge; and this in itself implies, as you will readily see, a possession of all knowledge, which, even in these days of free education and encyclopaedias, is a somewhat rare accomplishment.”

“(...) We are bound to go.”My answer was to rise from the table.“You are right, Holmes. We are bound to go.”He sprang up and shook me by the hand.“I knew you would not shrink at the last,” said he, and for a moment I saw something in his eyes which was nearer to tenderness than I had ever seen. The next instant he was his masterful, practical self once more.”