E1P14D
Demonstration — Part I
Latin
Cum Deus sit ens absolute infinitum de quo nullum attributum quod essentiam substantiæ exprimit, negari potest (per definitionem 6) isque necessario existat (per propositionem 11) si aliqua substantia præter Deum daretur, ea explicari deberet per aliquod attributum Dei sicque duæ substantiæ ejusdem attributi existerent, quod (per propositionem 5) est absurdum adeoque nulla substantia extra Deum dari potest et consequenter non etiam concipi. Nam si posset concipi, deberet necessario concipi ut existens; atqui hoc (per primam partem hujus demonstrationis) est absurdum. Ergo extra Deum nulla dari neque concipi potest substantia. Q.E.D.
English (Elwes 1883)
As God is a being absolutely infinite, of whom no attribute that expresses the essence of substance can be denied (by Def. vi.), and he necessarily exists (by Prop. xi.); if any substance besides God were granted, it would have to be explained by some attribute of God, and thus two substances with the same attribute would exist, which (by Prop. v.) is absurd; therefore, besides God no substance can be granted, or, consequently, be conceived. If it could be conceived, it would necessarily have to be conceived as existent; but this (by the first part of this proof) is absurd. Therefore, besides God no substance can be granted or conceived. Q.E.D.
Modern English
Since God is an absolutely infinite being, of whom no attribute expressing the essence of substance can be denied (E1D6), and who necessarily exists (E1P11), if any substance besides God could exist, it would have to be explained through some attribute of God, and then two substances of the same attribute would exist, which is absurd (E1P5). So no substance can exist outside God, and consequently none can be conceived either.
For if one could be conceived, it would necessarily have to be conceived as existing; but that, by the first part of this demonstration, is absurd. Therefore outside God no substance can exist or be conceived. Q.E.D.