Letters 66.11

But, since the virtues of plants and of animals are perishable, they are also frail and fleeting and uncertain. They spring up, and they sink down again, and for this reason they are not rated at the same value; but to human virtues only one rule applies. For right reason is single and of but one kind. Nothing is more divine than the divine, or more heavenly than the heavenly.

Seneca, Letters to Lucilius 66.11, translated by Richard Mott Gummere