It is of the highest importance in the art of detection to be able to recognise out of a number of facts which are incidental and which are vital.

My business is that of every other good citizen - to uphold the law.

Data!data!data!" he cried impatiently. "I can't make bricks without clay.

I have heard, Mr. Holmes, that you can see deeply into the manifold wickedness of the human heart.

I say, Watson,’ he whispered, ‘would you be afraid to sleep in the same room as a lunatic, a man with softening of the brain, an idiot whose mind has lost its grip?’ ‘Not in the least,’ I answered in astonishment. ‘Ah, that’s lucky,’ he said, and not another word would he utter that night.

Circumstantial evidence is occasionally very convincing, as when you find a trout in the milk, to quote Thoreau's example.

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. [...] It is a mistake to think that that little room has elastic walls and can distend to any extent. [...] It is of the highest importance, therefore, not to have useless facts elbowing out the useful ones.