Here appears to be a reference to Arist., De Gen. Ann. ii. 3, 11, πάσης μὲν ουν ψυχῆς δύναμις ἑτέρον σώματος ἐ& 231·ικε κεκοινωνηκέναι καὶ θειοτέρου τῶν καλουμένων στοιχείων· ὡς δὲ διαφέρουσι τιμιότητι αἱ ψυχαὶ καὶ ἀτιμί& 139· ἀλλήλων οὕτω καὶ ἡ τοιαύτη διαφέρει φύσις, and again, πνεῦμα…ἀνάλογον οὖσα τῷ τῶν ἄστρων στοιχεί& 251·.  On the fifth element of Aristotle cf. Cic., Tusc. Disp. i. 10.  Aristoteles…cum quatuor illa genera principiorum erat complexus, equibus omnia orirentur, quintam quandam naturam censet esse, equa sit mens.  Aug., De Civ. Dei xxii. 11. 2, and Cudworth’s Int. Syst. (Harrison’s Ed. 1845) iii. p. 465.  Hence the word “quintessence,” for which the Dictionaries quote Howard’s Translation of Plutarch, “Aristoteles hath put…for elements foure; and for a fifth quintessence, the heavenly body which is immutable.”  Skeat s. v. points out that “the idea is older than Aristotle:  cf. the five Skt. bhútas, or elements, which were earth, air, fire, and water, and æther.  Thus the fifth essence is æther, the subtlest and highest.”  It is evident that Milton had these theories in mind when he wrote (Par. Lost, iii. 716): “Swift to their several quarters hasted then The cumbrous elements, earth, flood, air, fire; And this ethereal quintessence of heaven Flew upward, spirited with various forms, That rolled orbicular, and turned to stars Numberless.”