Letters 72.11

Therefore, considering the great difference between those on the heights and those in the depths, and seeing that even those in the middle are pursued by an ebb and flow peculiar to their state and pursued also by an enormous risk of returning to their degenerate ways, we should not give ourselves up to matters which occupy our time. They should be shut out; if they once gain an entrance, they will bring in still others to take their places. Let us resist them in their early stages. It is better that they shall never begin than that they shall be made to cease. Farewell.

Seneca, Letters to Lucilius 72.11, translated by Richard Mott Gummere