Letters 89.9

The greatest authors, and the greatest number of authors, have maintained that there are three divisions of philosophy—moral, natural, and rational. The first keeps the soul in order; the second investigates the universe; the third works out the essential meanings of words, their combinations, and the proofs which keep falsehood from creeping in and displacing truth. But there have also been those who divided philosophy on the one hand into fewer divisions, on the other hand into more.

Seneca, Letters to Lucilius 89.9, translated by Richard Mott Gummere