Nothing in nature is contingent
Formal Statement
Nothing in the universe is contingent; all things are conditioned to exist and operate in a particular manner by the necessity of the divine nature.
In Plain Language
This is the ground rule for everything that follows. There is no "might have been." Every event, every state of affairs, follows from the nature of God/Nature with the same necessity that the properties of a triangle follow from its definition. When something looks contingent to us, that is a confession of ignorance, not a description of reality.
Why This Follows
This is our starting proposition. It follows from Spinoza's earlier proof that everything exists in God and depends on God (I.P15), and that God acts solely from the laws of his own nature (I.P16-17). If God is the only substance and everything is a mode of that substance, then nothing can be undetermined.
Necessity is the baseline of reality, not an occasional feature of it.
Connected Concepts
If nothing is contingent, what exactly are we doing when we say something "could have gone differently"?