P10: Each attribute of a substance must be conceived through itself.

Dem.: For an attribute is what the intellect perceives concerning a substance, as constituting its essence (by D4); so (by D3) it must be conceived through itself, q.e.d.

Schol.: From these propositions it is evident that although two attributes may be conceived to be really distinct (i.e., one may be conceived without the aid of the other), we still can not infer from that that they constitute two beings, or two different substances. For it is of the nature of a substance that each of its attributes is conceived through itself, since all the attributes it has have always been in it together, and one could not be produced by another, but each expresses the reality, or being of substance. So it is far from absurd to attribute many attributes to one substance. Indeed, nothing in nature is clearer than that each being must be conceived under some attribute, and the more reality, or being it has, the more it has attributes which express necessity, or eternity, and infinity. And consequently there is also nothing clearer than that a being absolutely infinite must be defined (as we taught in D6) as a being that consists of infinite attributes, each of which expresses a certain eternal and infinite essence.But if someone now asks by what sign we shall be able to distinguish the diversity of substances, let him read the following propositions, which show that in Nature there exists only one substance, and that it is absolutely infinite. So that sign would be sought in vain.