Al-Farabi's cosmology is essentially based upon three pillars: Aristotelian metaphysics of causation, highly developed Plotinian emanational cosmology and the Ptolemaic astronomy. In his model, the universe is viewed as a number of concentric circles; the outermost sphere or "first heaven", the sphere of fixed stars, Saturn, Jupiter, Mars, the Sun, Venus, Mercury and finally, the Moon. At the centre of these concentric circles is the sub-lunar realm which contains the material world. Each of these circles represent the domain of the secondary intelligences (symbolized by the celestial bodies themselves), which act as causal intermediaries between the First Cause (in this case, God) and the material world. Furthermore these are said to have emanated from God, who is both their formal and efficient cause.