Letters 15.6

Of course I do not command you to be always bending over your books and your writing materials; the mind must have a change,—but a change of such a kind that it is not unnerved, but merely unbent. Riding in a litter shakes up the body, and does not interfere with study; one may read, dictate, converse, or listen to another; nor does walking prevent any of these things.

Seneca, Letters to Lucilius 15.6, translated by Richard Mott Gummere