E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
in the Ptolemaic system of astronomy, was the tenth (not ninth) sphere, supposed to revolve from east to west in twenty-four hours, carrying with it all the other spheres. The eleven spheres are: (1) Diana or the Moon, (2) Mercury, (3) Venus, (4) Apollo or the Sun, (5) Mars, (6) Jupiter, (7) Saturn, (8) the starry sphere or that of the fixed stars, (9) the crystalline, (10) the primum mobile, and (11) the empyrean. Ptolemy himself acknowledged only the first nine; the two latter were devised by his disciples. The motion of the crystalline, according to this system, causes the precession of the equinoxes, its axis being that of the ecliptic. The motion of the primum mobile produces the alternation of day and night; its axis is that of the equator, and its extremities the poles of the heavens.
They pass the planets seven, and pass the fixed [starry sphere],
And that crystallin sphere and that First-Moved.
Milton: Paradise Lost, iii. 482.
Primum Mobile is figuratively applied to that machine which communicates motion to several others; and also to persons and ideas suggestive of complicated systems. Socrats was the primum mobile of the Dialectic, Megaric, Cyrenaic and Cynic systems of philosophy.