A short section of Aristotle's de generatione animalium embodies his final answer to the question how the faculties of soul are transmitted from parent to offspring. Aristotle here speaks in a tone which is dogmatic as well as enthusiastic; he is able to announce a new discovery. There is, he sets forth, in the sperma a peculiar substance (σῶμα) which has some connection with soul and differs in quality as the souls themselves differ in worth. This substance is identical with two of the entities mentioned in our title and ‘analogous’ to the third.
Πάσης μὲν οὖν ψυχῆς δύναμις ἑτέρου σώματος ἔοικε κεκοινωνηκέναι καὶ θειοτέρου τῶν καλουμένων στοιχείων ὡς δὲ διαφέρουσι τιμιότητι αἱ ψυχαὶ καὶ ἀλλήλων, οὕτω καὶ ἡ τοιαύτη διαφέρει φύσις. πάντων μὲν γὰρ ἐν τῷ σπέρματι ὑπάρχει ὅπερ ποιεῖ γόνιμα εἶναι τὰ σπέρματα, τό καλούμενον θερμόν. τοῦτο δ᾿ οὐ πῦρ πὐδὲ τοιαύτη δύναμίς ἐστιν, ἀλλὰ τὸ ἐμπεριλαμβανόμενον ἐν τῷ σπέρματι καὶ ἐν τῷ ἀφρώδει πνεῦμα καὶ ἡ ἐν τῷ πνεύματι φύσις, ἀνάλογον οὖσα τῶν ἄστρων στοιείῳ.
The sentences which follow state that fire has no generative or procreative power, yet such a power must be present in the Sun and in the θερμόν, the vital heat of living beings. Clearly, then, this θερμόν cannot be identical with the fire.
Nowhere else in the body of his preserved work does Aristotle establish this close connection between the vital heat, the pneuma, and the element of the stars, the so-called aether. These three concepts differ as much in their origin and past history as in their function and place within Aristotle's own physical or biological system. A brief sketch of them—skipping by necessity many significant episodes in the history of each—will suffice to make this clear.
What needs here to be said about the ‘element of the stars’ is indeed not much. It was Aristotle himself who added this element to the canonic four of the Empedoclean and Platonic tradition. The dialogue On Philosophy and the First Book On the Heaven secured it its place. It is divine, un-ageing, and unchanging, and yet a material element. Like the other elements it has its specific ‘natural motion’, to wit the circular, which makes it possible for Aristotle to explain by a physical ‘hypothesis’ the celestial motions for which Plato had resorted to the World-Soul. The place of this element is the entire heavenly region, extending from the First Heaven to the moon; below this, in the regions occupied by the four other elements, it is never to be found.