It is generally agreed that no activity can be successfully pursued by an individual who is preoccupied – not rhetoric or liberal studies – since the mind when distracted absorbs nothing deeply, but rejects everything which is, so to speak, crammed into it.
Denique inter omnes convenit nullam rem bene exerceri posse ab homine occupato, non eloquentiam, non liberales disciplinas, quando districtus animus nihil altius recipit, sed omnia velut inculcata respuit.
Original text (Latin)
Notes from Wikiquote
De Brevitate Vitae ("On the Shortness of Life", trans. C. D. N. Costa), Ch. 7