Sherlock Holmes and his personal reflections on many different matters.  Some of them are surprisingly insightful.

"I think there are certain crimes which the law cannot touch, and which therefore, to some extent, justify private revenge."

"Every problem becomes very childish when once it is explained to you."

"A dog reflects the family life. Whoever saw a frisky dog in a gloomy family, or a sad dog in a happy one? Snarling people have snarling dogs, dangerous people have dangerous ones."

Holmes: "I followed you."
Sterndale: "I saw no one."
Holmes: "That is what you may expect to see when I follow you."

"It is a capital mistake to theorize before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts."

"The world is big enough for us. No ghosts need apply."

"The world is full of obvious things which nobody by any chance ever observes."

"There’s the scarlet thread of murder running through the colourless skein of life, and our duty is to unravel it, and isolate it, and expose every inch of it."

"I am the last and highest court of appeal in detection."

"I assure you that the most winning woman I ever knew was hanged for poisoning three little children for their insurance money."

"I cannot agree with those who rank modesty among the virtues. To a logician all things should be seen exactly as they are, and to underestimate oneself is as much a departure from truth as to exaggerate one’s own powers."

"Some people without possessing genius have a remarkable power of stimulating it."

"The ways of Fate are indeed hard to understand. If there is not some compensation hereafter, then the world is a cruel jest."

"A man should keep his little brain attic stocked with all the furniture that he is likely to use, and the rest he can put away in the lumber room of his library, where he can get it if he wants it.

"Love is an emotional thing, and whatever is emotional is opposed to that true, cold reason which I place above all things. I should never marry myself, lest I bias my judgment."

Inspector Gregory: "Is there any other point to which you would wish to draw my attention?
Holmes: "To the curious incident of the dog in the nighttime."
Inspector Gregory: "The dog did nothing in the nighttime."
Holmes: "That was the curious incident."

"There is nothing more stimulating than a case where everything goes against you."

"I never make an exception. An exception disproves the rule."

"I can discover facts, Watson, but I cannot change them."

"After all, Watson, I am not retained by the police to supply their deficiencies."

"It is my business to know things. Perhaps, I have trained myself to see what others overlook."

"Where there is no imagination, there is no horror."

"You can never foretell what any one man will do, but you can say with precision what an average number will be up to. Individuals vary, but percentages remain constant."

"Is not all life pathetic and futile? Is not his story a microcosm of the whole? We reach. We grasp. And what is left in our hands at the end? A shadow? Or worse than a shadow--misery."

"Depend upon it, there is nothing so unnatural as the commonplace."