Spinoza's Ethics: I.A7: Non-existence

It is interesting to me that Spinoza chose to make this an axiom when a similar issue of the relationship of essence and existence is present in I.D1's definition of "self-caused." The difference seems to be the description of that essence and existence relationship as self-cause. So, it would appear that the axioms are more fundamental and might more appropriately precede the section of definitions.

In reviewing the difference between the definitions, the axioms and the propositions, it appears that Spinoza uses the axioms for the basic fabric of his thought. These axioms describe the relationships between things and things, between ideas and ideas and between things and ideas.

Quicquid ut non existens potest concipi, ejus essentia non involvit existentiam.

Translated as,

If a thing can be conceived as not existing, its essence does not involve existence.