If by the difference of their modifications--as substance is naturally prior to its modifications (Prop.

i.),--it follows that setting the modifications aside, and considering substance in itself, that is truly, (Deff.iii.and vi.), there cannot be conceived one substance different from another,--that is (by Prop.

iv.), there cannot be granted several substances, but one substance only.
Q.E.D.
PROP.VI.

One substance cannot be produced by another substance.
Proof .-- It is impossible that there should be in the universe two substances with an identical attribute, i.e.which have anything common to them both (Prop.

ii.), and, therefore (Prop.
iii.), one cannot be the cause of the other, neither can one be produced by the other.