P6: One substance cannot be produced by another substance.

Dem.: In nature there cannot be two substances of the same attribute (by P5), i.e. (by P2), which have something in common with each other. Therefore (by P3) one cannot be the cause of the other, or cannot be produced by the other, q.e.d.

Cor.: From this it follows that a substance cannot be produced by anything else. For in nature there is nothing except substances and their affections, as is evident from A1, D3, and D5. But it cannot be produced by a substance (by P6). Therefore, substance absolutely cannot be produced by anything else, q.e.d.

Alternatively: This is demonstrated even more easily from the absurdity of its contradictory. For if a substance could be produced by something else, the knowledge of it would have to depend on the knowledge of its cause (by A4). And so (by D3) it would not be a substance.