Quamlibet multa egerimus, quodam tamen modo recentes sumus ad id quod incipimus. quis non obtundi potest, si per totum diem cuiuscunque artis unum magistrum ferat? mutatione recreabitur sicut in cibis, quorum diversitate reficitur stomachus et pluribus minore fastidio alitur.
H. E. Butler's translation:
However manifold our activities, in a certain sense we come fresh to each new subject. Who can maintain his attention, if he has to listen for a whole day to one teacher harping on the same subject, be it what it may? Change of studies is like change of foods: the stomach is refreshed by their variety and derives greater nourishment from variety of viands.
Book I, Chapter XII, 5
De Institutione Oratoria (c. 95 AD)