There were three things in particular about which I persistently sought the reasons why they were such and not otherwise: the number, the size, and the motion of the circles. That I dared so much was due to the splendid harmony of those things which are at rest, the Sun, the fixed stars, and the intermediate space, with God the Father, and the Son, and the Holy Spirit.[9]

Kepler placed his triune God within the celestial sphere: the center as God, the sphere as Christ the Son, and the space between as the Holy Spirit. [10]  Throughout his attempts to deduce the distances of the planets a priori with Pythagorean principles and symmetry, Kepler maintained faith in his ability to uncover the virtus motrix (motive power) behind planetary motion and organization. In Mysterium he offers his first appeals to analogy between light and this virtus motrix, though they emerge only as weak “existence proofs” that allow Kepler to assume that the Sun’s influence weakens as a function of distance.[11] At this point Kepler’s knowledge of the virtus motrix was of course considerably less comprehensive than that of light, a gap he would close as his knowledge of each expanded. 

Kepler grounded his metaphysics of light in Plotinian-Neoplatonic emanationism, the ideology developed by Plotinus in which the source of all being is the cascading overflow or emanation of the divine One’s essence. This applies to all physical things, including lesser forms of being, which, like the divine, project a likeness or image onto their surroundings. The cascading effect extends infinitely, so that all physical things affect all other physical things at all times.[12] Plotinus linked light to his doctrine of emanation by comparing the rays of light of the Sun to the essence emanating from the One, calling each the species immateriata of its source. When Ficino adopted Plotinus’s emanationism in the 15th century, he further likened light to the soul, arguing that visible light unites the celestial and terrestrial realms in the same way the soul unites superior and inferior being.[13]  In the animistic universe of Ficino’s Renaissance, light was equally an animistic entity.[14]  Kepler adopted Plotinus’s emanationism and Ficino’s animism, and agreed with both that light, as a case of emanation readily accessible to human senses, was the doorway to understanding the universal principle of emanation.[15] However, Ficino, and Plotinus before him, struggled with Neoplatonism’s historical dilemma between unity and diversity, continuity and polarity; at some point classifications about light had to be made, but Neoplatonic ideology yielded no places on the continuum where clear lines could be drawn.[16] Plotinus and Ficino both found themselves caught between these opposite truths of the Neoplatonic tradition, and among the confusion failed to commit to definite answers to the crucial ontological questions regarding light’s corporeality, spirituality, and effect on its medium. 

Kepler harmonized Neoplatonism’s various contradictions and discontinuities concerning the qualities of light with his own empirical observations to formulate his theory of the nature of light. He presents its basic axioms in the first propositions of his optical treatise, Ad Vitellionem paralipomena quibus astronomiae pars optica traditur. Kepler starts by stating that the property of emanation is inherent to light, and clarifying that every point on a luminous body is a source of spherical emanation for an infinite number of lines, or geometric rays. His mystical foundations remain apparent: Kepler claims that this uniformly rectilinear propagation of light from all points on its source is due the teleological tendency of all things to imitate their Creator, and therefore strive for perfect, divine centricity. He also argues that light travels instantaneously to infinity as a two-dimensional geometrical surface that has “no matter, weight, or resistance.”[17]  Where Plotinus ambiguously rejected light’s corporeality, Kepler went further and defined the nature of light as mathematical, and light itself as mathematical substance.[18]  Kepler also observed that light’s concentration decreased as a function of the distance from its source, from which he derived the inverse-square law that he first states in Ad Vitellionem paralipomena. It is important to note that at this point Kepler states the effects of the inverse-square law in terms of the force’s density or concentration – the power is simply spread out, not lost, across a given distance.[19]  With these propositions Kepler succeeded in providing a stable empirical foundation from which to launch his study of light and consequently of the virtus motrix. Ultimately, however, his theory of the nature of light operated under the notion that one cannot define light, or God, for that matter; one can only declare what it is not and use comparison to support the negation. 

Kepler’s comparison of choice was analogy. He returned to the analogy between light and the virtus motrix almost obsessively across his body of work, introducing it in Mysterium and refining it throughout Astronomia nova and Epitome astronomia Copernicanae. In Ad Vitellionem paralipomena Kepler praises analogies as his “most faithful masters, acquainted with all the secrets of nature...they bring the solution of an infinity of cases lying between the extreme and the mean, and where they clearly present to our eyes the whole essence of the question.”[20]  Kepler extended his analogy to the point that light and the virtus motrix nearly became one and the same. In Chapter thirty-four of Astronomia he describes light as “an immaterial species of that fire which is in the body of the sun, so this power which enfolds and bears the bodies of the planets, is an immaterial species residing in the sun itself...the primary agent of every motion in the universe.”[21] The apparent similarities between both the quality and behavior of light and the virtus motrix were convincing, especially against the backdrop of Kepler’s Copernican, mystical, and theological convictions.

In Mysterium he points out a few key early observations concerning light and the virtus motrix: both emanate instantaneously from their source (essentially, in Kepler’s mind, the Sun for both), both are geometrical surfaces that do not exist in the intervening medium, and neither loses any power in travelling from its source to its illuminable or movable object.[22] Kepler did not limit himself to the similarities between light and the virtus motrix, however; he further analogized the Sun and the planets to sailors in a river, magnets, orators gazing at a crowd, and balanced scales to answer questions that analogy to light could not address.[23]  By allowing more familiar domains to inform his understanding of the virtus motrix, Kepler was able to pursue alignments and systems that would otherwise have been unapparent.  

Despite the persuasive capacity of the analogies, Kepler’s own observations forced him to reject the temptation to equate the virtus motrix with light and the Neoplatonic species immateriata it represented. Kepler’s first and most basic observation was to note that light interacts with only the surfaces of the bodies it illuminates, while his virtus motrix interacts with the “whole corporeality” of the planets it moves.[24] Kepler’s observation that light emanates spherically and the virtus motrix circularly from the Sun led to his analysis of light and the virtus motrix during a planetary eclipse: one planet may be eclipsed by another and therefore receive no visible light, yet the eclipsed planet does not stop moving and thus must still receive virtus motrix.[25]  Here Kepler’s theory of the virtus motrix appears primarily empirically justified and independent from his theory of light, but this independence is less a rejection and more a development of the theological and mystical motivations at the basis of Kepler’s theory.[26] Kepler did justice to his Lutheran and Neoplatonic beliefs by establishing the empirical truths behind the phenomena they glorified. His deeply held belief in the Sun’s dominance was at stake, and over time he had to revise some of his less critical, more abstract beliefs in favor of the evidence that validated his greater worldview of a heliocentric system. 

Clearly, highlighting similarities could get Kepler only so far in his efforts to find the “number, size, and motion of the circles,” so he attacked the inconsistencies between light and the virtus motrix with exemplum reasoning.[27] The exemplum is a more specific brand of analogy, geared toward illuminating universal laws and patterns of argument. Traditionally, exemplum “appeals to a similar or illustrative incident which is not intrinsically connected with the matter under discussion.”[28]  In the context of a regressus, exemplum may replace the negatiatio step in order to establish a genus to which all instances in question, or exempla, belong as species.[29]  Following the tradition of Lutheran natural philosophy, Kepler used logic and rhetoric to link physical and mathematical reasoning.[30]  His light analogies from Chapter thirty-four of Astronomia establish the physical basis for the mathematical exemplum-based inferences that validate the distance-velocity law – Kepler’s second law of planetary motion – in Chapter forty. This same rhetorical pattern of specific physical analogy to broad mathematical exemplum appears again in Chapter fifty-seven, which argues that reciprocation – Kepler’s first law – represents a natural law and is therefore part of the plan, God’s plan, for the world. For Kepler, successfully matching calculation to observation was not enough; the pattern inherent in the observations had to lend itself to physical explanation and therefore to classification as a universal law.[31]  

Kepler’s demonstration of light and the virtus motrix as two species of the same genus indicates the culmination of great epistemological and ontological modifications to the universe he first presented in Mysterium. In Mysterium, Kepler proposed an anima motrix (motive spirit) as the mover of the planets, a “single moving soul in the center of all the spheres, that is, in the sun.”[32] He later called his motive force the vis motrix (motive force) or, most consistently, the virtus motrix, referring to a more physical and tangible phenomenon. As his terminology became less animistic and more mechanical, so did his ontology. His initial motive spirit, derived directly from the theological and mystical nature of light, evolved with observation into a mechanical concept of the Sun’s influence.[33]  Kepler’s epistemology had to encompass the contradictory traditions that influenced his adoption of a physical universe, particularly Neoplatonism, Lutheranism, and the emerging mechanical philosophy of the 17th century. Embracing this intellectual ‘schizophrenia’ was the only way he could guarantee conclusions and demonstrate results to his audience’s – and his own – high standards of validity. 

Although he could not claim to define all the physical details of his virtus motrix, he could claim its existence, and at least some of its characteristics, by relating it to light and magnetism as a species of the genus of forces that attenuate with distance.[34]  Kepler imposed fundamentally Lutheran principles onto the Neoplatonic concept of emanation, which he used as a guide in his physical investigation of the mechanical motive force of the solar system. By recognizing the physical force responsible for the motions of the planets as a species of an established genus, Kepler could on theological grounds confirm its effects as laws of nature inherent in God’s plan for Creation. Throughout this investigation, Kepler’s theory of light developed alongside his changing concept of virtus motrix, each granting him the vital understanding he could not gain from the other.

Genevieve Gebhart is in the UW Honors Program and is majoring in International Studies in the Jackson School of International Studies with a concentration in environmental issues.  Ms. Gebhart was awarded a 2010 Library Research Award for a previous version of this essay.