Letters 85.13

But what folly it is, when the beginnings of certain things are situated outside our control, to believe that their endings are within our control! How have I the power to bring something to a close, when I have not had the power to check it at the beginning? For it is easier to keep a thing out than to keep it under after you have let it in.

Seneca, Letters to Lucilius 85.13, translated by Richard Mott Gummere