It will be seen from the foregoing passage how strong a hold the old Greek notion of an encircling limit had on the mind of Aristotle, and how he transformed it back from the high intellectual significance given to it by Plato into its original sense of a mere space-enclosing figure. And it will also be seen how he credits his spheres with a full measure of that moving power which, according to his rather unfair criticism, the Platonic Ideas did not possess. His astronomy also supplied him with that series of graduated transitions between two extremes in which Greek thought so much delighted. The heavenly bodies mediate between God and the earth; partly active and partly passive, they both receive and communicate the moving creative impulse. The four terrestrial elements are moved in the various categories of substance, quantity, quality, and place; the aether moves in place only. God remains ‘without variableness or shadow of a change.’ Finally, by its absolute simplicity and purity, the aether mediates between the coarse matter perceived by our senses and the absolutely immaterial Nous, and is itself supposed to be pervaded by a similar gradation of fineness from top to bottom. Furthermore, the upper fire, which must not be confounded with flame, furnishes a connecting link between the aether and the other elements, being related to them as Form to Matter, or as agent to patient; and, when the elements are decomposed into their constituent qualities, hot and cold occupy a similar position with regard to wet and dry. IV.