by Comparison: Lutheran
and Neoplatonic Conviction
Theory of Light
of Washington, Seattle
the course of his numerous publications, Johannes Kepler’s theory of
bridged the gap between his theological and Neoplatonic foundations and
revolutionary idea of a physical, causal astronomy. During his early
at Tubingen, Kepler encountered Michael Maestlin, the professor of
with whom he first studied Copernicanism. This introduction to the
of a heliocentric astronomy complemented Kepler’s belief that the sun
only possible source of the planets’ driving power and therefore had to
the center of the planetary system.
In addition to endorsing Copernicanism,
Maestlin also promoted the fundamentally Lutheran belief that the study
natural world yields knowledge of God’s plan for mankind, a race
equipped to discover the universe’s secrets through an innate knowledge
geometry that had been “inscribed on the human soul when it was
this fundamental Lutheran doctrine, Maestlin added an important caveat:
insisted that scientific accuracy, in astronomy especially, augmented
knowledge of God and providence.
This theological foundation was not mystical as much as it was a common
property of Lutheran belief and that of many other contemporary
the time. This “common sense” Lutheran theology provided an
legitimate basis upon which to connect the math-ematical arguments of
divine with the physical arguments of natural philosophy.
Kepler, this Lutheran worldview manifested itself in his relentless
study of the physical world
as the visible image of God.
examining the theological goals and arguments that define Kepler’s
must first understand the regressus
reasoning that dominates its structure. Foremost theologian of the
Philip Melanchthon profoundly influenced Kepler’s arguments in
by establishing regressus argument as the most rigorous
form of logical proof. A regressus consists of three sets of
arguments. The first, argument a
posteriori, derives description of an effect from
description of its
possible causes. The second, the
or the consideratio,
available alternatives, leaving one as the “true” cause. The final
argument a priori
assumes the new
“true” cause and from it deduces the original effect.
The Lutheran intellectual community accepted
the regressus method, and further
accepted a priori demonstration as
ideal in determining unique, true cause. 
regarded Copernicus’s De
as offering no more than a posteriori
demonstration to “save the appearances.” In Mysterium
he states that, “I had then reached the point of ascribing to this same
the motion of the Sun, but where Copernicus did so through mathematical
arguments, mine were physical, or rather, metaphysical.”
Kepler claimed that the arrangement of the
cosmos could have been proven logically using the idea of creation and
appealing to the “divine blueprint” of a
He goes on to state his rational goals and their very mystical
were three things in particular about which I persistently sought the
why they were such and not otherwise: the number, the size, and the
the circles. That I dared so much was due to the splendid harmony of
things which are at rest, the Sun, the fixed stars, and the
with God the Father, and the Son, and the Holy Spirit.
placed his triune God within the celestial sphere: the center as God,
sphere as Christ the Son, and the space between as the Holy Spirit.
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
weakens as a function of distance.
At this point Kepler’s knowledge of
motrix was of course considerably less comprehensive than that of
gap he would close as his knowledge of each expanded.
metaphysics of light in Plotinian-Neoplatonic emanationism, the
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
things, including lesser forms of being, which, like the divine,
likeness or image onto their surroundings. The cascading effect extends
infinitely, so that all physical things affect all other physical
things at all
Plotinus linked light to his doctrine of emanation by comparing the
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
light to the soul, arguing that visible light unites the celestial and
terrestrial realms in the same way the soul unites superior and
In the animistic universe of Ficino’s
Renaissance, light was equally an animistic entity.
Kepler adopted Plotinus’s emanationism and
Ficino’s animism, and agreed with both that light, as a case of
readily accessible to human senses, was the doorway to understanding
universal principle of emanation.
However, Ficino, and Plotinus before him, struggled with Neoplatonism’s
historical dilemma between unity and diversity, continuity and
some point classifications about light had to be made, but Neoplatonic
yielded no places on the continuum where clear lines could be drawn.
Plotinus and Ficino both found
themselves caught between these opposite
of the Neoplatonic tradition, and among the confusion failed to commit
answers to the crucial ontological questions regarding light’s
spirituality, and effect on its medium.
Neoplatonism’s various contradictions and discontinuities concerning
qualities of light with his own empirical observations to formulate his
of the nature of light. He presents its basic axioms in the first
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
that every point on a luminous body is a source of spherical emanation
infinite number of lines, or geometric rays. His mystical foundations
apparent: Kepler claims that this uniformly rectilinear propagation of
from all points on its source is due the teleological tendency of all
imitate their Creator, and therefore strive for perfect, divine
also argues that light travels instantaneously to infinity as a
geometrical surface that has “no matter, weight, or resistance.”
Where Plotinus ambiguously rejected light’s
corporeality, Kepler went further and defined the nature of light as
mathematical, and light itself as mathematical substance.
Kepler also observed that light’s
concentration decreased as a function of the distance from its source,
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
power is simply spread out, not lost, across a given distance.
With these propositions Kepler succeeded in
providing a stable empirical foundation from which to launch his study
and consequently of the virtus motrix.
Ultimately, however, his theory of the nature of light operated under
notion that one cannot define light, or God, for that matter; one can
declare what it is not
comparison to support the negation.
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
nova and Epitome
astronomia Copernicanae. In Ad
Vitellionem paralipomena Kepler
praises analogies as his “most faithful masters, acquainted with all
secrets of nature...they bring the solution of an infinity of cases
between the extreme and the mean, and where they clearly present to our
the whole essence of the question.”
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
bodies of the planets, is an immaterial species residing in the sun
itself...the primary agent of every motion in the universe.”
The apparent similarities between both the quality and behavior of
the virtus motrix were convincing,
especially against the backdrop of Kepler’s Copernican, mystical, and
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
power in travelling from its source to its illuminable or movable
object. Kepler did not limit himself to the
similarities between light and the virtus
motrix, however; he further analogized the Sun and the planets to
in a river, magnets, orators gazing at a crowd, and balanced scales to
questions that analogy to light could not address.
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.
capacity of the analogies, Kepler’s own observations forced him to
temptation to equate the virtus motrix
with light and the Neoplatonic species
immateriata it represented. Kepler’s first and most basic observation
to note that light interacts with only the surfaces of the bodies it
while his virtus motrix interacts
with the “whole corporeality” of the planets it moves.
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
light, yet the eclipsed planet does not stop moving and thus must still
Here Kepler’s theory of the virtus motrix appears primarily
empirically justified and independent from his theory of light, but
independence is less a rejection and more a development of the
mystical motivations at the basis of Kepler’s theory.
Kepler did justice to his Lutheran and Neoplatonic beliefs by
empirical truths behind the phenomena they glorified. His deeply held
the Sun’s dominance was at stake, and over time he had to revise some
less critical, more abstract beliefs in favor of the evidence that
his greater worldview of a heliocentric system.
similarities could get Kepler only so far in his efforts to find the
size, and motion of the circles,” so he attacked the inconsistencies
light and the virtus motrix with exemplum
The exemplum is a more specific brand
of analogy, geared toward illuminating universal laws and patterns of
Traditionally, exemplum “appeals to a
similar or illustrative incident which is not intrinsically connected
matter under discussion.”
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.
Following the tradition of Lutheran natural
philosophy, Kepler used logic and rhetoric to link physical and
His light analogies from Chapter thirty-four of
establish the physical
basis for the mathematical exemplum-based
inferences that validate the distance-velocity law – Kepler’s second
planetary motion – in Chapter forty. This same rhetorical pattern of
physical analogy to broad mathematical exemplum
appears again in Chapter fifty-seven, which argues that reciprocation –
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
pattern inherent in the observations had to lend itself to physical
and therefore to classification as a universal law.
demonstration of light and the virtus motrix as two species of
the same genus indicates the
culmination of great epistemological and ontological modifications to
universe he first presented 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
that is, in the sun.”
He later called his motive force the vis
motrix (motive force) or, most consistently, the virtus
referring to a more physical and tangible phenomenon.
As his terminology became less animistic and more mechanical, so did
ontology. His initial motive spirit, derived directly from the
mystical nature of light, evolved with observation into a mechanical
the Sun’s influence.
Kepler’s epistemology had to encompass
the contradictory traditions that influenced his adoption of a physical
universe, particularly Neoplatonism, Lutheranism, and the emerging
philosophy of the 17th century. Embracing this intellectual
the only way he could guarantee conclusions and demonstrate results to
audience’s – and his own – high standards of validity.
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
and magnetism as a species of the genus of forces that attenuate with
Kepler imposed fundamentally Lutheran
principles onto the Neoplatonic concept of emanation, which he used as
in his physical investigation of the mechanical motive force of the
system. By recognizing the physical force responsible for the motions
planets as a species of an
established genus, Kepler could on
theological grounds confirm its effects as laws of nature inherent in
plan for Creation. Throughout this investigation, Kepler’s theory of
developed alongside his changing concept of virtus
motrix, each granting him the vital understanding he could not gain
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
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