Letters 74.29

Virtue is just as great, even when it has retreated within itself and is shut in on all sides. For its spirit is no less great and upright, its sagacity no less complete, its justice no less inflexible. It is, therefore, equally happy. For happiness has its abode in one place only, namely, in the mind itself, and is noble, steadfast, and calm; and this state cannot be attained without a knowledge of things divine and human.

Seneca, Letters to Lucilius 74.29, translated by Richard Mott Gummere