Essential Thinkers #19 Isaac Newton, the Man Who Discovered Gravity

“This most beautiful system of the sun, planets and comets, could only proceed from the counsel and dominion of an intelligent and powerful Being.”
(Isaac Newton)

A mathematician and physicist, Sir Isaac Newton (1642-1727) produced work – philosophical to a degree – which served mainly as an impetus and basis for many of the philosophers of his and succeeding generations, including John Locke and Immanuel Kant, who both owed much to him. Newton’s principal work, the Philosophiae Naturalis Principia Mathematica contains his theory of gravity and laws of motion. His later work, the Opticks, is primarily concerned with optical physics but also contains speculations on mechanics, religion and morals. He was to be involved in a series of disagreements with Gottfried von Leibniz, initially over which of them was the first to invent the calculus, and later over the issue of the status of space and time.

The insight behind Newton’s physics was that the universe runs according to law-governed mechanical principles. This idea was to have a profound influence on John Locke, whose philosophy may be seen as the philosophical working out of Newton’s physical principles. Locke was determined to make sense of human understanding in a way consistent with Newtonian mechanics. As a result, he argued for a causal theory of perception and for a distinction between primary and secondary qualities of objects.

Emmanuel Kant, in similar fashion, recognized that everything in the phenomenal world had to conform to Newton’s principles, but that this order was for the most part imposed by the psychological apparatus of the mind. Kant’s philosophy gave support to Newton in the quarrel with Leibniz over whether space and time should be conceived of as absolute or merely as relations between objects. The debate seemed to have been won hands down by the Newtonians until the advent of Einstein’s relativistic physics.

Claiming that his method was empirical and inductive, rather than rationalist and deductive, Newton was also fond of criticizing Rene Descartes. It is thanks to Newton that empiricism began to enjoy a period of dominance over rationalist philosophy. However, Newton owed much to Descartes’ thought, and it is likely his own speculations could not have begun but for the work already undertaken by his rationalist predecessor.

Undoubtedly, Newton’s greatest achievement was his theory of gravity, from which he was able to explain the motions of all the planets, including the moon. Newton proved that every planet in the solar system at all times accelerates towards the sun. The acceleration of a body toward the sun is at a rate inversely proportional to the square of its distance from it. This led to Newton’s law of universal gravity: “every body attracts every other with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between them.” The law of universal gravity allowed Newton to predict all of the planetary motions, the tides, the movements of the moon and of the comets.

It was a striking achievement that would not be superseded until Albert Einstein, although even with the advent of Einsteinian relativity, Newton’s mechanics still holds good – and indeed is still used, on account of its simplicity, for predicting the movement of so-called ‘medium-sized’ objects – anything that is neither bigger than the solar system nor smaller than the eye can see. Newton’s work is profound and remarkable achievement in the history of human thought.

[Summarized from Philosophy 100 Essential Thinkers by Philip Stokes, 2012.]

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