One great thought--that of the unity of nature--lay behind them, but its significance was lost amid the phantasmagoria of Neo-Platonist exaltations. Hence the Bacchic fervours of Giordano Bruno took their inspiration; here was the groundwork of Spinoza's pantheism. Shelley's Demiorgon, felt as 'a living spirit,' seen as 'a mighty darkness,' descended lineally from that strange essence--formless, inarticulate, devoid of individual self-consciousness--which animated the submerged philosophy of Neo-Pagan times with the barren ardours of mysticism. The doctrine, in its original and more sober version, obtained memorable expression in Virgil's melodious hexameters:
'Principio cœlum, ac terras, camposque liquentes, Lucentemque globum lunæ, Titaniaque astra, Spiritus intus alit, totamque infusa per artus Mens agitat molem, et magno se corpore miscet.'
In Conington's rhymed version they run as follows:
'Know first, the heaven, the earth, the main, The moon's pale orb, the starry train, Are nourished by a soul, A bright intelligence, whose flame Glows in each member of the frame, And stirs the mighty whole.'
Kepler was no cosmogonist, but he aspired to found a 'physical astronomy,' and in his gropings for a mechanical power that might suffice to regulate the movements of the heavenly bodies, he stumbled upon a mode of action highly appropriate for the explanation of their growth. His ignorance of the laws of motion precluded him from the conception of velocities persistent in themselves, and merely deflected from straight into curved paths by a constant central pull. Hence he was driven to the twofold expedient of creating a whirling medium for maintaining the revolutions of the planets, and of supposing the sun to exercise a 'magnetic influence,' by which they were drawn into closed orbits. Here, then, central forces made a definitive entry on the astronomical stage, although with scarcely a discernible promise of their brilliant future. But it was otherwise with the clumsy machinery they helped to animate. Kepler's simple _modus operandi_, adopted, or more probably re-invented by Descartes, was published as an epoch-making discovery in his _Principia Philosophica_ (1644), and sprang under its new aspect into swift notoriety. The wide acceptance of the theory of vortices was at least in part due to the impressive largeness of its framework. Descartes left nothing out. The spacious scope of his speculations embraced all that was knowable--nature, animate and inanimate, life and time: