"I have seen those symptoms before," said Holmes, throwing his cigarette into the fire. "Oscillation upon the pavement always means an affaire de coeur."

It's a wicked world, and when a clever man turns his brain to crime it is the worst of all.

As I turned away, I saw Holmes, with his back against a rock and his arms folded, gazing down at the rush of the waters. It was the last that I was ever destined to see of him in this world. - Watson.

Anything seems commonplace, once explained.

horses: dangerous on both ends and crafty in the middle

Life, it turns out, is infinitely more clever and adaptable than anyone had ever supposed.

The larger crimes are apt to be the simpler, for the bigger the crime, the more obvious, as a rule, is the motive.