Isaac Newton

Isaac Newton (4 January 1643 – 20 March 1727) was an English mathematician, astronomer, and physicist/natural philosopher who is widely recognized as one of the most influential scientists of all time. Newton’s most notable contributions were made to the fields of physics, mathematics, and scientific method, which were so groundbreaking that he is currently considered to be one of the most important physicists in modern Western history.1 Philosophers of science credit Newton’s revolutionary theory of gravity and his experimental approach to conducting natural philosophy as outlined in his major work, Philosophiæ Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy or simply The Principia), whose principles became central to the mosaic of late 18th and 19th century science.1 Some consider The Principia to be the work that initially created physics as its own scientific field separate from the umbrella of metaphysics and philosophy. 1

Historical Context

When Isaac Newton began his studies at Cambridge University's prestigious Trinity College in 1661, more than a century had passed since Nicolaus Copernicus (1473-1543) had proposed a heliocentric cosmology in his 1543 De Revolutionibus Orbium Coelestium (On the Revolutions of Heavenly Spheres). It had been fifty years since Galileo Galilei (1564-1642) had published his observations with the telescope in 1610, which uncovered dramatic evidence favoring the Copernican system. Around the same time, Johannes Kepler (1571-1630)published his laws of planetary motion, indicating that the planets revolved around the sun on elliptical paths, replacing the circular motion and complex epicycles of Copernicus and Ptolemy.2pp. 1-7 According to Westfall, "by 1661 the debate on the heliocentric universe had been settled; those who mattered had surrendered to the irresistible elegance of Kepler's unencumbered ellipses, supported by the striking testimony of the telescope, whatever the ambiguities might be. For Newton, the heliocentric universe was never a matter in question".2p. 6 A planetary Earth that rotated on its axis and revolved around the sun was incompatible with the accepted Aristotelian physics. The community of the time was engaged with the question of how it could be that the Earth itself was in motion through space, and with the question of how one could hope to gain reliable knowledge in the face of the failure of Aristotelian scholastic knowledge accepted for centuries.

Newton’s education at Cambridge was classical, focusing on Aristotelian rhetoric, logic, ethics, and physics. Bound to Aristotelian scholasticism by statutory rules,3p. 33 the curriculum had changed little in decades.2pp. 81-904 Like many of the more ambitious students, Newton distanced himself from classical metaphysics and instead studied the works of the French natural philosopher René Descartes(1596-1650) on his own. By 1664, Newton is known to have read the 1656 Latin edition of Descartes' Opera Philosophica, a one volume compilation of Descartes' major works.4 Descartes had died just over a decade prior, and these works had first been published within the preceding thirty years. They were gaining in popularity and by about 1680 would become the accepted centerpiece of the Cambridge curriculum, as they also would in Paris by 1700.5p. 190 Newton is known to have been profoundly influenced by Descartes views of space, matter, and God, and by commentaries on Descartes by Henry More (1614-1687). 6

While Newton’s physics and philosophy were heavily influenced by Descartes’ ideas, they also became a challenge to what had, by the time he published, become the new Cartesian orthodoxy. When Newton published his magnum opus, Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy)in 1687, Descartes' views had been accepted at Cambridge. The title of Newton's work suggests he intended it to be in dialog with Descartes' Principia Philosophiae (Principles of Philosophy) published in 1644.1 Descartes was the most prominent member of a community of corpuscularist thinkers, who maintained that visible objects were made of unobservably tiny particles, whose relations and arrangement were responsible for the properties of visible bodies. In this mechanical natural philosophy, particles influenced one another only by direct physical contact, which was the cause of all motion, and ultimately all change.7

Aristotle (384-322 BCE) had explained the properties of visible bodies in terms of their form, rather than in terms of the arrangement of their constituent parts. He maintained that heavy objects, composed of the element earth, tended towards their natural place; the center of the universe. The concept of a sphere of earth at rest in the center of the universe was central to Aristotle's entire cosmology. Motion in the terrestrial and celestial realms were seen as fundamentally different.8 Descartes' theories allowed for a movable planetary Earth, and celestial motions that weren't different in kind from terrestrial motions. They explained gravity, in qualitative terms, as due to a swirling vortex of particles around the Earth, which pushed things towards its center. In accord with Copernican heliocentrism, Descartes posited that a larger vortex surrounded the sun, with the smaller planetary vorticies caught in a larger solar vortex.97 In Newton's time, major champions of the mechanical natural philosophy included Christiaan Huygens (1629-1695) and Gottfried Wilhelm Leibniz (1646-1716), who was to become a major rival of Newton's.

Descartes saw the ultimate justification of knowledge claims to lie with human reason and the absence of doubt. He relied on classical methods of theorizing and conjectured hypotheses in order to construct scientific propositions.1 Such a rationalist approach to knowledge was also championed by Baruch Spinoza (1632-1677), Nicolas Malebranche (1638-1715), and by Gottfried Wilhelm Leibniz.10 But, by the early 17th century, experimental researchers like Galileo Galilei and Robert Boyle (1627-1691) had begun to elaborate and practice a very different approach to knowledge based on experimentation and extensive use of mathematics. Following the inductive methodology advocated by Francis Bacon(1561-1626), they maintained that theoretical principles emerged from experimental data by a process of inductive generalization. However, there were also dissenters like Newton's contemporary Christiaan Huygens, who believed that most experimental work involved formulating hypotheses about unobservable entities, which were tested by their observable consequences. This was an early form of hypothetico-deductivism. Newton rejected Cartesian rationalism, and argued that the Cartesians did not sufficiently employ mathematics and experimentation in their work. He rejected the method of hypotheses outright. 111 He supported inductivism, and held epistemological views similar to those of his contemporary and friend John Locke(1632-1704), who maintained that all knowledge came from experience.12

Major Contributions

Hakob Barseghyan's lecture on Newtonian Worldview

Hakob Barseghyan's lecture on Newtonian Worldview

Newton on Mathematics and Natural Philosophy

Newton's two most important works of natural philosophy were the Principia, published in 1687, which dealt with his theories of motion and universal gravitation, and Opticks: or, A Treatise of the Reflexions, Refractions, Inflexions, and Colours of Light which was published in 1704 and dealt with his theories of light and color. 13 More than Descartes, Newton made mathematics central to the conduct of natural philosophy, by producing a general mathematical theory of the motion of bodies. 1 He posited three mathematical laws of motion, together with a law of universal gravitation. Changes in the state of motion of objects were caused by forces acting on them. Quantities of force and amounts of matter were measurable. The laws specified the mathematical relationship between the acceleration experienced by an object, the quantity of matter composing it, and the magnitude of the forces acting on it. 4

In contrast with the Cartesian mechanical philosophy, in Newton’s physics, material objects were not required to be in direct contact in order to influence each other's motion. Forces could act at a distance. To explain both falling bodies and the motions of the moon and planets, Newton posited a gravitational force that acted as the inverse square of the distance between objects. He claimed to have derived this relationship from Kepler's observational laws of planetary motion. The works of Ptolemy, Copernicus, and Kepler used the mathematical language of geometry in their descriptive accounts of celestial motions. In the Principia Newton likewise presented his arguments geometrically. Newton sought not simply to describe celestial motions, but to explain these motions in terms of gravitational forces acting between bodies. In order to do this, Newton invented a new branch of mathematics, integral and differential calculus. Calculus deals with mathematical quantities that are continuously changing, such as the magnitude and direction of gravitational forces acting on an orbiting body. 144 He developed the basic concept of calculus during 1665-6, while Cambridge University was closed due to a plague. 15p. 10

Although Newton circulated manuscripts much earlier, he did not actually publish his work on calculus until the first decade of the eighteenth century. 15p. 20 He is thus co-credited with inventing calculus alongside his contemporary and rival natural philosopher, Leibniz.15pp. 13-14 Eventually, in the 18th century, mathematicians Jacob Hermann and Leonhard Euler expressed Newton’s laws of motion using Newton's own technique of calculus and the symbolic expressions that Leibniz had developed.4p. 29

Newton on Methodology

Prior to the publication of The Principia, the philosophy of motion and change in the universe was largely a theoretical and non-mathematical enterprise. The dominating methodological approach both in the Aristotelian-scholastic and Cartesian natural philosophy, was one in which truths about the natural world were proposed as conjectural hypotheses. They were often deduced from fundamental metaphysical principles that were deemed evidently true by human reason 110. Influenced by the more experimental and mathematically oriented methodologies of Bacon, Galileo, and Boyle, Newton explicitly rejected this method of hypotheses, and instead demanded that all propositions be deduced from the observed phenomena and then converted into general principles via induction 11116. In the second edition of the Principia, Newton states:

I have not as yet been able to deduce from phenomena the reason for these properties of gravity, and I do not feign hypotheses. For whatever is not deduced from the phenomena must be called a hypothesis; and hypotheses, whether metaphysical or physical, or based on occult qualities, or mechanical, have no place in experimental philosophy. In this experimental philosophy, propositions are deduced from the phenomena and are made general by induction. The impenetrability, mobility, and impetus of bodies and the laws of motion and law of gravity have been found by this method. And it is enough that gravity should really exist and should act according to the laws that we have set forth and should suffice for all the motions of the heavenly bodies and of our sea.17p. 276

Newton called his method the experimental philosophy, because theories about the behavior of empirical objects can only be refuted via experimental procedures.16 He expressed the core beliefs from which he derived his method in a set of four “rules for the study of natural philosophy,” which he stated in book III of The Principia as follows:

1. No more causes of natural things should be admitted than are both true and sufficient to explain their phenomena
2. Therefore, the causes assigned to natural effects of the same kind must be, so far as possible, the same
3. Those qualities of bodies that cannot be intended and remitted (i.e. qualities that cannot be increased and diminished) and that belong to all bodies on which experiments can be made should be taken as qualities of all bodies universally
4. In experimental philosophy, propositions gathered from phenomena by induction should be considered either exactly or very nearly true notwithstanding any contrary hypothesis, until yet other phenomena make such propositions either more exact or liable to exceptions.17pp. 794-796

Out of these four rules a new, engaged method for conducting science emerged that stood in stark contrast to the previous passive and theoretical Cartesian and Aristotelian-scholastic methods. Propositions formulated based on observations of the natural world and placed back into the natural world to be tested empirically.16 The calculus became deeply incorporated into the experimental method, as it was used to mathematically calculate empirical predictions from natural laws, and then evaluate how exactly the prediction matched the observed reality. Newton claimed to have derived his law of universal gravitation using this method as applied to Kepler's laws of planetary motion. In the Cartesian natural philosophy, centripetal force had already been defined as the agent that pulled the moon towards the Earth, keeping its orbit circular rather than linear. Newton appealed to rules 1) and 2) to claim that the centripetal force, and the force that compelled objects to move downwards towards the Earth, were merely two different expressions of the same thing. Newton then went on to apply the third rule, and argue that this force, which he called gravity, must be a universal property of all material objects. From here, he went on to argue for the unification of superlunary and sublunary phenomena.18pp. 183-184

Historical research indicates that the scientific community did not use Newton's own criteria in evaluating his work. Newton's theories did not become accepted outside of England until after its prediction of the oblate spheroid shape of the Earth was confirmed by expeditions to Lapland and Peru. Thus, Newton's theories became accepted via a hypothetico-deductive method based on confirmed novel predictions that distinguished it from the rival Cartesian vortices, rather than via Newton's own inductive methodology. 5p. 48-491911 According to McMullin, Newton's methodology ran contrary to the consensus that had been emerging among natural philosophers of his time, in favor of hypothesis. 11 Christiaan Huygens and John Locke are known to have taken the experimental philosophy, if not necessarily the full content of Newton’s theories, to heart.1

Criticism

To proponents of the mechanical philosophy, it was important that all motion in the universe be given a cause involving direct physical contact, even if this amounted to a larger gap between theory and experimental evidence.1 Newton's contemporary, Leibniz, in particular was concerned that the theory of gravity as a regression in natural philosophy, as Newton could not give a mechanistic account of the source of gravity.

Although many natural philosophers in the 17th century were convinced by Newton’s views on the the proper method of conducting science, many were not willing to abandon the Cartesian mechanical philosophy.

Publications

Here are the works of Newton included in the bibliographic records of this encyclopedia:

• Newton (1999): Newton, Isaac. (1999) The Principia: Mathematical Principles of Natural Philosophy. University of California Press.
• Newton (1952): Newton, Isaac. (1952) Opticks or A Treatise of the Reflections, Refractions, Inflections & Colors of Light. Dover Publications.
• Newton (1704): Newton, Isaac. (1704) Opticks: or, A Treatise of the Reflexions, Refractions, Inflexions and Colours of Light. Prince's Arms in St. Paul's Churchyard. Retrieved from https://archive.org/details/opticksortreatis00newt.
• Newton (1687): Newton, Isaac. (1687) Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy). Pepys, London.

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Related Topics

Methodology