Nothing is destroyed except by an external cause
Formal Statement
Nothing can be destroyed except by a cause external to itself. The definition of any thing affirms its essence but does not negate it; so long as we regard a thing in itself, we find nothing that could destroy it.
In Plain Language
This looks deceptively simple but it is load-bearing. A thing's definition says what it is, not what it isn't. There is nothing inside a stone, a person, or an idea that amounts to a self-destruct button. Destruction always comes from outside. If you are falling apart, something external is doing the work. This will matter in two steps when Spinoza derives the drive to persist.
Why This Follows
From the setup: each thing is a determinate mode of substance (I.P25 Corollary). Its essence is what it is, and by definition an essence affirms being — it does not contain its own negation. Any force of destruction must therefore originate beyond the thing itself.
Self-destruction is impossible; every threat to a thing's existence comes from outside.
Does this mean that self-destructive behaviour in humans is, in Spinoza's framework, always ultimately caused by external forces? What would those forces be?