>>>>>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. Q.E.D.
<<<<<VI. Corollary--Hence it follows that a substance cannot be
produced by anything external to itself. For in the universe
nothing is granted, save substances and their modifications (as
appears from Ax. i. and Deff. iii. and v.). Now (by the last
Prop.) substance cannot be produced by another substance,
therefore it cannot be produced by anything external to itself.
Q.E.D. This is shown still more readily by the absurdity of the
contradictory. For, if substance be produced by an external
cause, the knowledge of it would depend on the knowledge of its
cause (Ax. iv.), and (by Deff. iii.) it would itself not be
substance.