P4: Two or more distinct things are distinguished from one another, either by a difference in the attributes of the substances or by a difference in their affections.

Dem.: Whatever is, is either in itself or in another (by A1), i.e. (by D3 and D5),outside the intellect there is nothing except substances and their affections. Therefore, there is nothing outside the intellect through which a number of things can be distinguished from one another except substances, or what is the same (by D4), their attributes, and their affections, q.e.d.