Substance is prior to its modifications
Formal Statement
Substance is by nature prior to its modifications. Because modes exist in substance and are conceived through it (Def.5), while substance exists in itself and is conceived through itself (Def.3), substance must be both ontologically and conceptually prior to any mode.
In Plain Language
This is the first real move of the argument, and it follows almost immediately from the definitions. Modes need substance the way ripples need water — you cannot have the modification without the thing being modified. Substance, on the other hand, does not need any particular mode in order to be. Priority here is not about time; it is about dependence.
Why This Follows
By the definition of mode (gs-03), modes exist in and are conceived through something else — namely substance. By the definition of substance (gs-01), substance is in itself and conceived through itself. So substance is logically and ontologically prior: modes depend on it, but it does not depend on them.
Substance comes first in the order of being and understanding; modes depend on it, not vice versa.
If substance is prior to all its modes, does that mean substance could exist with no modes at all?