Reason perceives things under a form of eternity
Formal Statement
It is in the nature of reason to perceive things under a certain form of eternity (sub quadam aeternitatis specie). Reason regards things not as contingent but as necessary, and this necessity is the very necessity of the eternal nature of God.
In Plain Language
When reason grasps something, it grasps it as necessary and timeless. The Pythagorean theorem does not become true on Tuesdays — it holds eternally, because it follows from the nature of triangles. Reason, by its very structure, lifts us out of the temporal flux of imagination into contact with the eternal necessity that runs through everything. This is not mysticism; it is simply what happens when you understand causes rather than just registering effects.
Why This Follows
Step 1 (df-01) established that nothing is contingent. Step 6 (df-06) identified reason as the faculty that produces adequate ideas. This step connects them: reason's adequate perception is necessarily a perception of necessity, and that necessity is eternal. Reason does not add eternity to things — it sees the eternity that was always there.
Through reason, necessity becomes intelligible rather than merely imposed.
Connected Concepts
What does it mean to understand something "under a form of eternity"? Is Spinoza talking about timelessness, or about something more specific?