We may, indeed, indulge in sport and jest, but in the same way as we enjoy sleep or other relaxations, and only when we have satisfied the claims of our earnest, serious task.
Ludo autem et ioco uti illo quidem licet, sed sicut somno et quietibus ceteris tum, cum gravibus seriisque rebus satis fecerimus.
Original text (Latin)
Notes from Wikiquote
Book I, section 103