Letters 97.16

Good luck frees many men from punishment, but no man from fear. And why should this be if it were not that we have ingrained in us a loathing for that which Nature has condemned? Hence even men who hide their sins can never count upon remaining hidden; for their conscience convicts them and reveals them to themselves. But it is the property of guilt to be in fear. It had gone ill with us, owing to the many crimes which escape the vengeance of the law and the prescribed punishments, were it not that those grievous offences against nature must pay the penalty in ready money, and that in place of suffering the punishment comes fear. Farewell.

Seneca, Letters to Lucilius 97.16, translated by Richard Mott Gummere