Human beings are unique in al-Farabi's vision of the universe because they stand between two worlds: the "higher", immaterial world of the celestial intellects and universal intelligibles, and the "lower", material world of generation and decay; they inhabit a physical body, and so belong to the "lower" world, but they also have a rational capacity, which connects them to the "higher" realm. Each level of existence in al-Farabi's cosmology is characterized by its movement towards perfection, which is to become like the First Cause, i.e. a perfect intellect. Human perfection (or "happiness"), then, is equated with constant intellection and contemplation.