Letters 74.19

For this reason foresight must be brought into play, to insist upon a limit or upon frugality in the use of these things, since licence overthrows and destroys its own abundance. That which has no limit has never endured, unless reason, which sets limits, has held it in check. The fate of many cities will prove the truth of this; their sway has ceased at the very prime because they were given to luxury, and excess has ruined all that had been won by virtue. We should fortify ourselves against such calamities. But no wall can be erected against Fortune which she cannot take by storm; let us strengthen our inner defences. If the inner part be safe, man can be attacked, but never captured.

Do you wish to know what this weapon of defence is?

Seneca, Letters to Lucilius 74.19, translated by Richard Mott Gummere