That philosophy which proceeds on experiments, and deduces the laws of nature, and the properties and powers of bodies, and their action upon each other, from sensible experiments and observations. The business of experimental philosophy is to inquire into and investigate the reasons and causes of the various appearances and phenomena of nature, and to make the truth or probability thereof evident to the senses, by plain, undeniable, and adequate experiments, representing the several parts of the grand machinery and agency of nature. Sir Isaac Newton, the greatest master in the science, lays down four rules by which to guide our inquiries into nature. 1st, More causes of natural things are not to be admitted than are both true and sufficient to explain the phenomena; 2d, and therefore, of natural effects of the same kind, the same causes are to be assigned, as far as it can be done, as of respiration in man and beasts; of light in a culinary fire and in the sun; and of the reflection of light in the earth and in the planets. 3d, The qualities of natural bodies, which cannot be increased or diminished, and agree to all bodies in which experiments can be made, are to be reckoned as the qualities of all bodies whatever; thus because extension, divisibility, hardness, impenetrability, mobility, the vis inertiae, and gravity, are found in all bodies which fall under our own cognizance or inspection, we may justly conclude they belong to all bodies whatsoever, and are, therefore, to be esteemed the universal and original properties of all natural bodies. 4th, In natural philosophy, propositions collected from the phenomena by induction, are to be deemed (notwithstanding contrary hypotheses) either exactly or very nearly true, till other phenomena occur by which they may be rendered either more accurate, or liable to exception.
This ought to be done lest arguments of induction should be destroyed by hypothesis. These four rules of philosophizing are premised by Sir Isaac Newton to his third book of the Principia, and more particularly explained by him in his Optics, where he exhibits the mode of proceeding in philosophy.