Letters 56.11

Men think that we are in retirement, and yet we are not. For if we have sincerely retired, and have sounded the signal for retreat, and have scorned outward attractions, then, as I remarked above, no outward thing will distract us; no music of men or of birds can interrupt good thoughts, when they have once become steadfast and sure.

Seneca, Letters to Lucilius 56.11, translated by Richard Mott Gummere