I am but mad north-north-west. When the wind is southerly, I know a hawk from a handsaw.
Act II, Scene 2 · Hamlet
Context
Hamlet tells Rosencrantz and Guildenstern that his madness is conditional and that he can still distinguish between things when the conditions are right.
Analysis
The specificity of 'north-north-west' and 'when the wind is southerly' makes madness sound like a weather pattern—intermittent and directional rather than total. This precision is itself evidence of sanity, since a truly mad person would not be able to map the borders of their condition so clearly.
Essay Tip
Use this to argue that Hamlet is in control of his madness—he is telling his friends directly that it is a strategic performance, though the metaphor is cryptic enough that they might miss the admission.