[156] [cap.30] What, then, is the proof that they had not entirely completed this building? First of all, the manifest notoriety of the fact. For it is impossible for even so slight a portion of the earth to touch the heaven, by reason of the cause beforementioned, that no centre can ever touch the circumference; in the second place, because the aether (αἰθής) is sacred fire and an unquenchable flame, as its very name shows, being derived from αἴθω, to burn, which is a synonymous word with καίω.