Our knowledge of ourselves and external things is mostly inadequate
Formal Statement
The ideas of modifications of the human body, insofar as they are referred only to the human mind, are not clear and distinct but confused. We can only have very inadequate knowledge of the duration of our own body and of external things.
In Plain Language
Here is why we feel as if things are contingent even though they are not. Our sensory experience is partial: we perceive effects without grasping their full causes. When you see the sun, your idea of it reflects how your body was affected, not the sun's actual nature. This confusion is the source of the illusion of contingency. We call things "possible" or "accidental" precisely because we do not see the necessity behind them.
Why This Follows
Steps 1-3 (df-01 through df-03) established that nothing is contingent. A question naturally arises: if everything is necessary, why does the world seem so unpredictable? This step answers: because most of our ideas are inadequate. The feeling of contingency is an epistemological artefact, not an ontological fact.
Ignorance of causes produces the illusion of contingency.
Connected Concepts
Can you think of a time when something seemed random until you understood the cause — and then it seemed obvious? What changed: the event, or your knowledge?