Paraphrase
Things are naturally contrary — that is, cannot exist in the same subject — insofar as one is capable of destroying the other.
Contraries cannot coexist when one can destroy the other
Claims
Interpretation
If two contrary natures could coexist in one thing, that thing would contain something capable of destroying it, which contradicts III.P4.
In Plain Language
Imagine trying to hold fire and ice in the same hand at the same time. If they are genuine contraries — one capable of annihilating the other — they cannot stably occupy the same subject. This is the clean-up step. Combined with ce-05 it means: a thing not only lacks internal self-destruction, it actively resists the presence of anything that could destroy it. The ground is now prepared for the conatus.
Why This Follows
Directly from ce-05: if nothing is destroyed except by an external cause, then if two contraries could inhabit the same thing, that thing would contain its own destruction — contradicting ce-05. So contraries are excluded from coexistence.
A thing opposes whatever could destroy it; contraries are structurally expelled.
Connected Concepts
If contraries cannot coexist in the same subject, how should we understand inner conflict — say, wanting to quit smoking while still craving a cigarette?