Mathematical Principles of Natural Philosophy

...which nature herself bestows. But when they take the liberty to assume whatever unknown shapes and sizes of parts they please, along with uncertain positions and motions; and when they even invent certain occult fluids occult fluids: subtle, invisible substances (like the "ether") that many 17th-century thinkers imagined filled the universe to explain how forces moved through space that freely permeate the pores of bodies, possessed of an all-powerful subtlety and agitated by hidden motions; they then slide into dreams, neglecting the true constitution of things. This truth is sought in vain from deceptive conjectures, when it can scarcely be tracked down even through the most certain observations. Those who take the foundation of their speculations from hypotheses hypotheses: in Newton's era, this term specifically referred to assumptions or "guesses" not yet proven by observation—even if they proceed thereafter with the utmost accuracy according to mechanical laws—may be said to compose an elegant and beautiful fable original: "fabulam"; Cotes is likely taking a jab at René Descartes, whose "vortex theory" Newton sought to replace, but it is a fable nonetheless.

There remains, therefore, a third class: those who profess experimental philosophy experimental philosophy: the scientific method based on observation and repeatable experiments rather than pure logic or "armchair" reasoning. These thinkers indeed wish to derive the causes of all things from the simplest possible principles; however, they assume nothing as a principle that has not yet been proven by phenomena. They do not devise hypotheses, nor do they admit them into physics, except perhaps as questions whose truth may be debated. They proceed by a double method: analysis and synthesis. They deduce the forces of nature and the simpler laws of those forces from certain selected phenomena through analysis; from these, they then provide the constitution of the remaining things through synthesis. This is that far superior way of philosophizing which our most celebrated author deemed worthy to be embraced above all others. This alone he judged worthy of his effort to cultivate and adorn. Of this method, he gave the most illustrious example: namely, the explanation of the system of the world original: "mundani systematis"; this refers to the structure and mechanics of the solar system most successfully deduced from the theory of gravity. Others had suspected or imagined that the power of gravity existed in all bodies; he was the first and only one who could demonstrate it from appearances and place it upon a firm foundation by his excellent speculations.

I am well aware, indeed, that some men of great name, being more occupied by certain prejudices than is fair, have found it difficult to agree with this new principle and have repeatedly preferred uncertainties to certainties. It is not my intention to pick at their reputation; rather, kind reader, I wish to set before you in a few words the facts from which you may bear a fair judgment yourself.

Therefore, to begin the argument from the simplest and nearest things, let us look for a moment at what the nature of gravity is in earthly objects, so that we may then proceed more safely to the celestial bodies, which are located at a very great distance from—