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The Principia, arguably the most important book published in modern European history, began by offering the reader three basic principles, which have come to be known as Newton's three laws of motion:
- 1. Every body continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it.
- 2. The change in motion is proportional to the motive force impressed, and is made in the direction of the straight line in which that force is impressed.
- 3. To every action there is always opposed an equal reaction. Even today, these three laws are the basic axioms on which physics rests, and the first principles that every high school physics student learns.
From this opening, Newton proceeded to the meat of his book, the principle of gravitation and its role in the solar system. He showed how his inverse square law worked perfectly with Kepler's elliptical orbits; how planets are deflected into orbit around the sun by the pull of the sun's gravity, and how the same principle can be used to explain the orbit of the moon and of Jupiter's moons; he demonstrated that Descartes' theory of vortices lacked the same explanatory power. Working from Halley's research on the subject, he declared that comets transcribed orbits around the sun just as planets did; he calculated the mass of each planet; he used the pull of the sun's gravity to account for the flattening of the Earth at the poles and the bulge at the equator; he used the gravitational pull of the moon and sun to explain the ocean tides. In his account, the entire universe was held together in a web of gravitational pulls, acting on every star, planet, moon and comet; thus Newton rendered the whole universe explainable by a law--subject to the insight of mathematics and the human mind.
It was, as Newton admitted, a purely mathematical analysis. How gravity worked was not explained; the work did not posit anything analogous to Descartes' swirling particles. But Newton's system did correspond to actual calculations, and he proved its hypotheses beyond a reasonable doubt: he backed very point, every argument, by rigorous mathematic and experimental evidence. The work contained no theorizing, no wild speculation--only a succession of rigorously supported assertions that showed a dazzled continent how the heavens functioned.
The Principia fell upon European intellectual circles like a bombshell. The first edition, written and published in Latin, sold out rapidly, and a second edition was not issued until 1713, making copies rare and much sought- after. And yet while the scientific community quickly acknowledged the importance of what Newton had done, wide acceptance of his arguments did not come immediately: in France, where Descartes still reigned supreme, scholars objected that Newton's force of gravity had no logical basis--that it was an essentially supernatural notion; thus for decades these thinkers clung to the theory of vortices, abandoning it only when the great (French) philosopher Voltaire championed Newton in the 1730s. Similarly, in Germany, the brilliant mathematician Leibniz also took issue with Newton's failure to explain how gravity worked. In England, meanwhile, the major complaint was quite different: critics worried that Newton's 'clockwork' cosmos left no room for divine intervention, and accused his text of promoting deism, or even atheism.
Thus Newton decided to publish a second edition, in 1713, in which he endeavored to placate his critics. To Leibniz and the French, he admitted that he did not know what enabled gravitational pull to act across the void of interstellar space--though he protested that he had never claimed to understand the nature of gravity, but only the mathematical laws that governed its operation. Meanwhile, for the devout, he appended a section on the role of God to the second edition, in which he insisted that "this most beautiful system of the sun, planets, and comets, could only proceed from the counsel and dominion of an intelligent and powerful being." To his mind, the order that he found in the cosmos proved, rather than challenged, the idea of a benevolent and all- powerful creator: he saw his scientific work as a way to glorify God by revealing the greatness, complexity, and underlying order of creation. God himself was beyond human understanding: "as a blind man has no idea of colors," Newton wrote, "so we have no idea of the manner by which the all-wise God perceives and understands all things." But the structure of the universe provides a clue, enabling us to "know (God) . . . by his most wise and excellent contrivances of things, and final causes." As for the idea that science could lead to atheism, Newton dismissed it brusquely: "Blind metaphysical necessity, which is certainly the same always and everywhere, could [not] produce [the] variety of things" found on our diverse and ever-surprising world.
