Letters 71.31

Just as wool takes up certain colours at once, while there are others which it will not absorb unless it is soaked and steeped in them many times; so other systems of doctrine can be immediately applied by men’s minds after once being accepted, but this system of which I speak, unless it has gone deep and has sunk in for a long time, and has not merely coloured but thoroughly permeated the soul, does not fulfil any of its promises.

Seneca, Letters to Lucilius 71.31, translated by Richard Mott Gummere