Pierre Gassendi, Mercurius in Sole Visus

               

The French philosoper and scientist Pierre Gassendi (1592-1655) was
instrumental in laying the foundations of modern scientific thought.
He is most widely remembered for having resurrected the ancient Greek
concept of atomism, which had been held by Democritus (465-375 BC) 
and Epicurus (341-270 BC).  (Aristotle cited both Democritus and 
an earlier individual named Leucippus (c.450 BC) as adherents of the
atomistic philosophy, but Epicurus asserted that Leucippus was a 
fictional personage, perhaps the creation of Democritus, similar to
the suggestions that Socrates was the creation of Plato.)  None of
the 72 works atrributed to Democritus by the historian Diogenes
Laertius has survived, but there are several surviving works of
Epicurus in which the early doctrine of atomism is described.  In
addition, we have the epic poem "De rerum natura" composed by the
Roman poet and philosopher Lucretius (94-50 BC), describing and
elaborating on the atomism of Epicurus.

The atomistic philosophy seems to have been originally an attempt to
reconcile the Eleatic notion of unchanging oneness with the evident
plurality and dynamism of the world.  To combine these two seemingly
incompatible paradigms, Democritus proposed a world consisting of 
an empty void and an infinite number of finite, indivisible, and 
unchanging "atoms", all composed of the same incorruptable essence,
differing only in shape, size, and in their orientations and
arrangements within the void.  The importance of this conception of
the world can hardly be over-stated.  It seeks to account for all
the variety of secondary properties and qualities of our experience
solely by the set of possible spatial and dynamical configurations
of simple elemental entities.  Interestingly, although the early 
atomists held that the number of atoms of any given shape was infinite
(as was the extent of the void), the number of different shapes was
held to be finite (although very large).  In principle, then, it was
possible to catalog all the distinct types of elementary atoms that
comprise the world.

The uniformity of essence of the atoms making up qualitatively 
different macroscopic substances, such as water and iron, was a very
profound and consequential idea, in opposition to the notions of
Aristotle (for example), who held that there were multiple different
"essences", responsible for the different behaviors of different
substances, e.g., air rises because of it's essential nature.  The
atomist cannot avail himself of such teleological explanations, so
he must consider the different possible configurations and arrangements
of essentially identical atoms in order to find explanations for the
different behavior of different macroscopic substances.  For example,
water flows easily because its atoms are smooth, whereas the atoms
compirising a block of iron have hooks that link them together 
in rigid formations.  Likewise, lighter substances consist of 
configurations of atoms that are not packed closely together, and
so the contain empty spaces.  The atomistic philosophy has obvious
materialistic tendancies, since it tempts us to apply the same sort
of reductionist analysis to the behavior of living things, including
even ourselves.

In view of this, it might seem surprising that Gassendi, who took
holy orders, was an official of the Catholic Church, and a very devout
Christian, would have been attracted to atomism.  It's been suggested
that his advocacy of Epicurianism was more a reaction against the
prevailing Scholasticism of the day and the teachings of Aristotle 
than a positive conviction.  Of course, this begs the questions of
WHY he felt it necessary to reject the telelogical explanations of
Aristotle in the first place.  Presumably he was dissatisfied with
them, and this led him to see whether it was possible to reconcile
a more satisfactory system of explanations with Christianity.  His
earnest efforts to describe atomism within the context of Christian
teachings have led some later materialists to disparage and even
ridicule him.  For example, Karl Marx made the memorable comment that 
Gassendi had "tried to put a nun's habit on the body of Lais". (Lais
was the name of a famous "courtesan" - to put it politely - of ancient
Greece.)

Whatever his motivations, the fact remains that Gassendi sought out
and found the concept of atomism, and adopted it as a more suitable
framework (than Aristotle's) for explaining the workings of nature,
and this was of profound significance for the later progress of the
scientific revolution.  For example, it's known that Newton read 
Gassendi, and indeed we can see an attempt to "dress Lais" (just as
Gassendi had done) in Newton's famous statement 

  "It seems probable to me that God in the beginning formed 
   matter of solid, massy, hard, impenetrable movable particles,
   of such sizes and figures, and in such proportion to space, 
   as most conduced to the end for which He formed them... and...
   that the changes of corporeal things are to be placed only 
   in the various separations and new associations and motions
   of these permanent particles..."

If the revival of the atomistic paradigm had been Gassendi's only
accomplishment, it would have earned him a leading place among the
founders of modern scientific thought, but in fact he was also an
early (perhaps even the first) proponent of another, equally profound
and significant idea, namely, the modern concept of inertia.  This 
insight is usually attributed to Galileo, and with some justification,
because it was Galileo who founded the quantitative science of dynamics
based on the principle of equivalence between uniformly moving systems 
of reference.  However, there always remained in Galileo's thought
certain remnants of the teleological notions of Aristotle, such as
the belief that the "natural unforced motion" of some entities (e.g.,
the planets) was circular, on the grounds that this was the most
perfect conceivable motion.  His attachment to purely circular motion
actually led Galileo to reject Kepler's theory of elliptical orbits,
and prevented him (Galileo) from seeing gravitation as a universal
force whose effect is to continuously divert the planets from their
"true" natural (inertial) motions, which are strictly linear.  Of
course, there is a certain irony in the fact that, three centuries
later, all free-fall motion, including the motions of the planets, 
came once again to be seen as natural (unforced) and inertial in 
Einstein's general theory of relativity.  

Nevertheless, it was crucial for the development of physics in the
17th century to first gain a clear understanding of natural inertial
motion in isolated empty regions, and this required scientists to 
abstract away the incidental effects of gravitation on the Earth's 
surface, and formulate inertial motion in modern terms, as the 
tendancy for objects to continue at rest or in any state of uniform
motion in a straight line.  (On close inspection, this proposition 
is admitedly circular, as Mach and other subsequently pointed out,
but it forms a surprisingly robust basis for doing physics, provided
we naively accept the intuitive concepts of straightness and 
uniformity.)  It was precisely this clear conception of inertia, 
later codified as Newton's First Law of Motion, that was first 
articulated by Pierre Gassendi.  In this most fundamental respect,
Gassendi saw more clearly than Galileo the shape and character of 
modern dynamics, which forms the basis for all of modern physics.

Gassendi's conception of inertia was closely related to his atomism, 
which entails a denial of special qualities or "natures" for different
kinds of matter, some (celestial objects) preferring circular motion 
and some (terrestial objects) preferring linear motion.  Gassendi's 
world view automatically commited him to a more egalatarian view of 
matter, since the substance comprising every atom is identical.

The empiricism and moderate skepticism advocated by Gassendi was 
also very important for the development of modern science.  Perhaps
his most famous statement was that 

       "There is nothing in the intellect which has not 
        been in the senses." 

This is a sophisticated proposition, denying the possibility of 
apriori notions of any kind, and asserting that even the most 
primitive elements of rational thought are ultimately derived from,
and based on, experience.  It was this spirit that motivated the
famous experiment performed by Gassendi, in which a cannonball was
dropped from the mast of a ship sailing uniformly in a straight line,
and the ball was observed to land at the foot of the mast, giving
a concrete demonstration of Galileo's principle of relativity.  
Admittedly, Gassendi didn't apply strict empiricism consistently,
since at other times he admitted the possibility of general abstract
ideas that have no foundation in the senses, but his outlook was 
definitely more solidly empirical than the prevailing Scholasticism
of his immediate predecessors, and more thoroughly materialistic than
the dualism of Descartes.

In addition to his profound contributions to the conceptual framework
of modern science, Gassendi also advanced the practice of astronomy, 
and in 1631 he became the first human being in history to witness an 
eclipse other than the familiar solar and lunar eclipses.  It's worth
remembering that the inferior planets are visible easily from the
Earth only when they are not too near the line of sight to the Sun,
and in ancient times Venus (for example) was actually regarded as
two different objects.  When Venus appeared as the evening star in
the western sky, it was called Hesperus by the ancient Greeks, whereas
when it appeared as the morning star in the eastern sky it was called
Phosphorus.  The connection between these two objects was obscure.
However, following the Copernican (and Keplerian) model of the solar
system, the planets all revolve around the Sun, and the orbits of 
Venus and Mercury are inside the orbit of the Earth, with calculable
periods of revolution.  From this we can deduce specific times at
which the planet Mercury must pass directly between the Earth and 
the Sun, and it should be visible as a small dot passing over the 
face of the Sun.  This is exactly what Gassendi observed on the 7th
of November, 1631.  He gave an account of this observation in his
paper "Mercurius in sole visus".  Along with Galileo's observation 
of the phases of Venus, the transit of Mercury was among the first
pieces of direct evidence in favor of the Copernican over the 
Ptolemaic (geocentric) conception of the solar system.

Coincidentally, it was observations of the transits of Mercury that,
nearly 300 years later, provided the first strong evidence for in
favor of Einstein's general theory of relativity, which, as noted
above, ironically redeemed Galileo's intuition that the planetary 
motions are inertial (i.e., geodesics).