Mary Watson: Every movement I made was entirely random, every new personality just on the roll of a dice!

Sherlock Holmes: Mary, no human action is ever truly random. An advanced grasp of the mathematics of probability, mapped onto a thorough apprehension of human psychology and the known dispositions of any given individual, can reduce the number of variables considerably. I myself know of at least 58 techniques to refine a seemingly infinite array of randomly generated possibilities down to the smallest number of feasible variables. But they’re really difficult, so instead I just stuck a tracer on the inside of the memory stick.

Mary Watson: Oh, you b*stard! You b*stard!

Sherlock Holmes: I know, but your face!

Mary Watson: “The mathematics of probability”?

Sherlock Holmes: You believed that.

Mary Watson: “Feasible variables”?

Sherlock Holmes: Yes, I started to run out about then.

From Sherlock – Season 4 Episode 1: ‘The Six Thatchers’ (4×01)