'I had,' said he, 'come to an entirely erroneous conclusion which shows, my dear Watson, how dangerous it always is to reason from insufficient data.'

'Men die of the diseases which they have studied most,' remarked the surgeon, snipping off the end of a cigar with all his professional neatness and finish. 'It's as if the morbid condition was an evil creature which, when it found itself closely hunted, flew at the throat of its pursuer. If you worry the microbes too much they may worry you. I've seen cases of it, and not necessarily in microbic diseases either. There was, of course, the well-known instance of Liston and the aneurism; and a dozen others that I could mention.'

'There's no need for fiction in medicine,' remarks Foster... 'for the facts will always beat anything you fancy.'

ROSS: Is there any point to which you would wish to draw my attention?
HOLMES: To the curious incident of the dog in the night-time.
ROSS: The dog did nothing in the night-ime.
HOLMES: That was the curious incident.

I could not help laughing at the ease with which he explained his process of deduction. 'When I hear you give your reasons,' I remarked, 'the thing always appears to me to be so ridiculously simple that I could easily do it myself, though at each successive instance of your reasoning I am baffled, until you explain your process. And yet I believe that my eyes are as good as yours.'
'Quite so,' he answered, lighting a cigarette, and throwing himself down into an arm-chair. 'You see, but you do not observe. The distinction is clear. For example, you have frequently seen the steps which lead up from the hall to this room.'
'Frequently.'
'How often?'
'Well, some hundreds of times.'
'Then how many are there?'
'How many! I don't know.'
'Quite so! You have not observed. And yet you have seen. That is just my point. Now, I know that there seventeen steps, because I have both seen and observed.'

When a doctor does go wrong he is the first of criminals. He has nerve and he has knowledge.

When we think how narrow and devious this path of nature is, how dimly we can trace it, for all our lamps of science, and how from the darkness which girds it round great and terrible possibilities loom ever shadowly upwards, it is a bold and a confident man who will put a limit to the strange by-oaths into which the human spirit may wander.

When you have eliminated the impossible, whatever remains, however improbable, must be the truth.

[Holmes]: The temptation to form premature theories upon insufficient data is the bane of our profession.