Mercurius in Sole Visus

The French philosopher 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 attributed 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 incorruptible 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 comprising 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 tendencies, 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 teleological 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  tendency for objects to continue at rest or in any state of uniform motion in a straight line.  (On close inspection, this proposition  is admittedly 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 (terrestrial objects) preferring linear motion.  Gassendi's  world view automatically committed him to a more egalitarian 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  a priori 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).

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