The Empyrean is the schoolmen's answer to a question Aristotle's cosmos could not ask: what is outside the outermost sphere? For Aristotle, strictly nothing, not even void; the universe simply ended. Christian doctrine, however, required a heaven, the dwelling of God, the angels and the saints, and locating it inside the machinery of moving spheres seemed to subject beatitude to physics. The solution, developed by the twelfth and thirteenth centuries and adopted by Aquinas's teachers, was a tenth heaven of pure light, caelum empyreum, fiery in splendor rather than heat, created on the first day, filled with glory from its beginning, and, decisively, immobile.
The immobility is the doctrine. Motion, in the Aristotelian analysis, is the signature of incompleteness, a thing moving toward what it lacks; the spheres turn because they desire. The Empyrean desires nothing, lacking nothing, and therefore rests, and its rest is not idleness but plenitude, the stillness of arrival. From this followed its odd relation to place: it contains the whole moving universe while being contained by nothing, so that, as the axiom ran, the heaven of heavens is a place only in the sense that it embraces all places while needing none. Later writers drew the mystical corollary: to ascend through the spheres is to pass from time toward its rim, and the crossing into the Empyrean is the crossing from time into eternity, not a further distance but a change of state, the point where the journey stops being travel.