would explain the centripetal factor of the planets’ orbits, “yet the transverse motions by which they revolve in their several orbs required a divine Arm to impress them according to the tangents of their orbs.” Since this did not necessarily include the rotational motion of the planets, Newton added “that the diurnal rotations of the Planets could not be derived from gravity but required a divine power to impress them.”32
This cosmological argument had a mechanical counterpart in the fact that, according to Newton’s second law of motion, any change of motion was proportional to the magnitude of a force impacting a body; both the impact and the resulting direction would be in a straight line. How could rotation originate from the impact of just one force?33
For reasons like these Immanuel Kant introduced a second “original” force besides gravitation into the fabric of the universe in his daring Allgemeine Naturgeschichte und Theorie des Himmels of 1755: the repulsive force. He blunted the audacity of this addition to Newton’s mechanics by arguing that “these two forces are both equally certain, equally simple, and at the same time equally primal and universal. Both are taken from Newtonian philosophy. The first is now an incontestably established law of nature. The second, which Newtonian science perhaps cannot establish with as much clarity as the first, I here assume only in the sense which no one disputes, that is, in connection with the smallest distributed particles of matter, as, for example, in vapours.”34 To show the primordial interplay of these forces, Kant imagined the world “on the immediate edge of creation,” when the universe was filled with matter at rest for a time “which lasts but an instant.”35 Since atoms were created with different specific weights, the heavier ones attracted the lighter ones and began to form “gobs” (Klumpen.)36 All matter would collapse into one big gob were it not for the repulsive force that inflected the straight path of onrushing matter and sent it into an orbit around the central, that is, heaviest body. Applied to the formation of the solar system, the interaction of these two forces explained why all planets orbited around the sun in one plane—the central mystery for Newton in his exchange with Bentley. They were all remnants of the initial cloud of matter that had first collapsed on, and then been flung from, the heaviest gob in one part of the universe, the sun. The same had happened in countless other corners of the universe.
This “nebula hypothesis” was a theory with extraordinary explanatory power, justly famous for its range and daring: in one fell swoop it explained the origin of empty space (as the consequence of matter contracting), the spherical form of celestial bodies (as the consequence of the simultaneous rush of particles on a common center and the resulting rotation) and the common plane of all orbits in the solar system, resulting in a fully mechanical cosmogony.37 But subtly it also reversed the question of the origin of rotation. In a later chapter, “Concerning the Origin of Moons and the Axial Rotation of the Planets,” Kant makes the much-needed distinction between orbital motion (“Zirkelbewegung”) and axial rotation (“Achsendrehung”) and explains the origin of the latter as the result of particles impacting the already forming body, off-center and from opposite sides, and thereby keeping it spinning.38 The diameter of the planet serves as a lever on which the particles exert opposite, yet equal translational force. Kant’s hypothesis anticipates here the notion of torque as the product of the length of a lever arm and two opposite perpendicular forces: he argues, for example, that Jupiter rotates faster than smaller planets (like Mars), which can be explained only by the fact that it has a larger diameter: “If the axial rotation were an effect of an external cause [e.g., God’s twisting motion], then Mars would have to have a more rapid axial rotation than Jupiter, for the very same power of movement affects a smaller body more than a larger one. We would quite correctly be surprised at this, since all the orbital movements diminish with distance from the mid-point, but the speeds of the rotations increase with the distance.”39 What needs to be explained, this theory implies, is not rotational motion (for it is a natural effect of the self-creation of the material universe) but its cessation. Why, then, do some planets rotate around their axes and others, like the moon, not? In the Universal Natural History Kant promises to solve this problem in his answer to one of the Academy prizes, and indeed he does so in a small essay of 1754 with a very long title.40 There he shows that the orbit of the moon around the earth is the result of the earth’s greater mass having dragged the satellite by its (now evaporated) aqueous surface and finally locked it into its present synchronous rotation. The same will happen, Kant knows, to the earth once the moon’s drag on its oceans overcomes its rotational momentum.
The great conceptual problem of Kant’s history of the heavens, immediately seized upon by the next generation of natural philosophers, lies in the assumption of two original forces.41 A system based on two principles is unable to close itself off; it remains susceptible to the charge of contingency, to that which cannot be anticipated or grounded. This uncertainty is expressed in Kant’s cosmogony by the curious temporal assignations of the “immediate edge of creation” and the “instant” of equilibrium—Kant cannot further account for their occurrence, nor can he explain why attractive forces operate first and repulsion follows later. According to the Romantic philosophers of nature, who succeeded Kant and who acknowledged their debt to his writings on natural science while eagerly moving away from his mechanistic thought, the principal motions cannot interact in such a desultory fashion, and, what is more, they must follow from principles that are valid for both natural and intellectual phenomena. Otherwise, the relation of nature to our understanding would remain inexplicable, and the system would again suffer from contingency. Rotation, this implies, cannot be the result of two supervening forces but has to originate together with the system itself, and it has to have a subjective manifestation.
This, at least, was the way the most scientifically inclined idealist philosopher, F. W. J. Schelling, argued. He neither accepted the contingent relation between attraction and repulsion at the origin of rotation nor countenanced the separation of mechanical causes from organic (and ultimately intellectual) ones. In his own rewriting of Plato’s Timaios, Von der Weltseele (1798), he advanced the notion that the world was a “universal organism” and that its motions and interactions were governed by two forces that formed a polarity: one was the other of the other, neither existed by itself. Nature would not coalesce into solid phenomena if the tendency to expand were not checked by a “returning motion.” These two polar forces—whose avatars, among others, were positive and negative magnetism and electricity, chemical affinity and repulsion, physiological irritability and sensibility—animated the universal organism and kept its soul in constant motion. Since Plato and Aristotle had already argued that motion originating from the soul was superior to all others, and above all that it could initiate rotation, Schelling could spend comparatively little energy on explaining the origin of rotation. If everything potentially rotated, it was rectilinear motion that required explanation.42
Schelling’s Naturphilosophie underwent a few metamorphoses before he expanded his perspective even further and considered—in his Philosophy of Revelation—creation and the becoming of God as a process of rotational gestation. His followers and successors kept their focus on the primacy of rotation in the explanation of the natural world. Lorenz Oken, one of the most influential teachers of Romantic natural philosophy, declared confidently: “God is a rotating globe. The world is God rotating. All motion is rotational, and there is everywhere no straight motion any more than there is a single line of straight surface. Everything is comprehended in ceaseless rotation. . . . Straight motion is only the mechanical; such, however, exists not through itself. The more a body moves in a straight direction, the more mechanical and ignoble it is.”43 Hegel interpreted the solar system as a kind of cosmic mind, where the sun represented subjectivity in its most abstract form as self-relation (because rotation was motion that related only to itself); the moons, which circled their center of gravitation without rotating, were entirely other-related; and the planets, including the earth, combined both motions by rotating and orbiting at the same time. This figure of an initial rotation that exteriorizes itself in its component motions recurs at various junctures in Hegel, whose philosophical system in its totality has been described as depicting a multiplicity of spheres rotating around a common center.44
