Philo Judaeus De Opificio Mundi 36

The incorporeal world then was already completed, having its seat in the Divine Reason; and the world, perceptible by the external senses, was made on the model of it; and the first portion of it, being also the most excellent of all made by the Creator, was the heaven, which he truly called the firmament, as being corporeal; for the body is by nature firm, inasmuch as it is divisible into three parts; and what other idea of solidity and of body can there be, except that it is something which may be measured in every direction? therefore he, very naturally contrasting that which was perceptible to the external senses, and corporeal with that which was perceptible only by the intellect and incorporeal, called this the firmament.