אתיקה, חלק I, משפט 5

If several distinct substances be granted, they must be distinguished one from the other, either by the difference of their attributes, or by the difference of their modifications (Prop. iv.). If only by the difference of their attributes, it will be granted that there cannot be more than one with an identical attribute. 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.