Isaac Newton (1642-1727)
Principia (1687)

The Original Electronic Text at the web site the Internet Modern History Sourcebook.

The publication of Newton's Principia in 1687 marks both an end and a beginning. Because his treatise effectively solved the problems posed by Copernicus's theory, it brought the first great "Scientific Revolution" to a close. Newton developed a mechanics and astronomy that integrated the work of his predecessors like Galileo and Kepler and that could account for matter in motion on a moving earth and in cosmic systems. His work is often termed a "synthesis" because the same simple mathematical laws governed terrestial and celestial phenomena alike, as contrasted with the ancient and medieval system wherein sharp distinctions were made between the heavens and the sub-lunar world. The Principia also embodied and articulated a philosophy and method of science, incorporating elements of Bacon's inductive and empirical method and Descartes's deductive and mathematical method, but also embracing a moderate scepticism that refused to take theory beyond empirical evidence and treated scientific propositions as provisional and probable, not absolute and certain. The Principia, together with his later work, The Optics, also stand at the beginning of a new era in Western intellectual history. Although many scientists, especially on the Continent, continued for decades to favor Descartes's theories over Newton's, scientists of the eighteenth and nineteenth century looked to Newton's work for ideas and models. Newton's theories also influenced non-scientists. The philosophes of the Enlightenment were eager to apply Newton's scientific method to the study of society, government, and humanity, and they viewed Newton's work as a kind of symbol of the potential of human reason.

1. How does Newton's method compare with those of Bacon and Descartes? with those of ancient and medieval scientists?
2. What does rule four imply about certainty of scientific propositions?
3. Why does Newton refuse to "frame a hypothesis"?

The Rules of Reasoning in Philosophy


We are to admit no more causes of natural things, than such as are both true and sufficient to explain their appearances.

To this purpose the philosophers say, that Nature does nothing in vain, and more is in vain, when less will serve; for Nature is pleased with simplicity, and affects not the pomp of superfluous causes.


Therefore to the same natural effects we must, as far as possible, assign the same causes.

As to respiration in a man, and in a beast; the descent of stones in Europe and in America; the light of`our culinary fire and of the sun; the reflection of light in the earth, and in the planets


The qualities of bodies, which admit neither intension nor remission of degrees, and which are found to belong to all bodies within reach of our experiments, are to be esteemed the universal qualities of all bodies whatsoever.

For since the qualities of bodies are only known to us by experiments, we are to hold for universal, all such as universally agree with experiments; and such as are not liable to diminution, can never be quite taken away. We are certainly not to relinquish the evidence of experiments for the sake of dreams and vain fictions of our own devising; nor are we to recede from the analogy of Nature, which is wont to be simple, and always consonant to itself. . . .


In experimental philosophy we are to look upon propositions collected by general induction from phenomena as accurately or very nearly true, notwithstanding any contrary hypotheses that may be imagined, till such time as other phenomena occur, by which they may either be made more accurate, or liable to exceptions.

This rule we must follow that the argument of induction may not be evaded by hypotheses.

General Scholium

Hitherto we have explained the phenomena of the heavens and of our sea by the power of gravity, but have not yet assigned the cause of this power. This is certain, that it must proceed from a cause that penetrates to the very centers of the sun and planets, without suffering the least diminution of its force; that operates not according to the quantity of the surfaces of the particles upon which it acts (as mechanical causes used to do), but according to the quantity of the solid matter which they contain, and propagates its virtue on all sides to immense distances, decreasing always as the inverse square of the distances. Gravitation towards the sun is made up out of the gravitations towards the several particles of which the body of the sun is composed; and in receding from the sun decreases accurately as the inverse square of the distances as far as the orbit of Saturn, as evidently appears from the quiescence of the aphelion of the planets; nay, and even to the remotest aphelion of the comets, if those aphelions are also quiescent.

But hitherto I have not been able to discover the cause of those properties of gravity from phenomena, and I frame no hypotheses; for whatever is not deduced from the phenomena is to be called an hypothesis; and hypotheses, whether metaphysical or physical, whether of occult qualities or mechanical, have no place in experimental philosophy. In this philosophy particular propositions are inferred from the phenomena, and afterwards rendered general by induction. Thus it was that the impenetrability, the mobility, and the impulsive force of bodies, and the laws of motion and of gravitation, were discovered. And to us it is enough that gravity foes really exist, and act according to the laws which we have explained, and abundantly serves to account for all the motions of the celestial bodies, and of our sea.

Isaac Newton, The Mathematical Principles of Natural Philosophy, trans. A. Motte (London, 1729). [Capitalization and spelling have been modernized.]

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(c)Paul Halsall Aug 1997