Will cannot be called a free cause
Formal Statement
Will cannot be called a free cause, but only a necessary cause. Will is a particular mode of thinking and therefore (by I.P28) requires a cause that determines it, which in turn requires a cause, and so on to infinity.
In Plain Language
People carve out an exception for human choice: "Sure, rocks follow laws, but my decisions are free." Spinoza blocks this move at the level of God himself. Even God's will, if we use that word, is determined by the necessity of his own nature. So your will is not an escape hatch from causality — it is one more link in the causal chain.
Why This Follows
Step 1 (df-01) established universal necessity. This step applies it specifically to will, closing the most common objection: that the mind somehow stands outside the causal order.
Choice is not an exception to necessity — it is an instance of it.
Connected Concepts
When you deliberate about a decision, does the feeling of "weighing options" prove that your will is uncaused, or could that feeling itself be determined?