Letters 113.4

Each living thing must have a separate substance; but since all the things mentioned above have a single soul, consequently they can be separate living things but without plurality. I myself am a living thing, and a man; but you cannot say that there are two of me for that reason. And why? Because, if that were so, they would have to be two separate existences. This is what I mean: one would have to be sundered from the other so as to produce two. But whenever you have that which is manifold in one whole, it falls into the category of a single nature, and is therefore single.

Seneca, Letters to Lucilius 113.4, translated by Richard Mott Gummere