The philosophy which Mr Newton in his Principles and Opticks has pursued is experimental; and it is not the business of experimental philosophy to teach the causes of things any further than they can be proved by experiments. We are not to fill this philosophy with opinions which cannot be proved by phenomena. In this philosophy hypotheses have no place, unless as conjectures or questions proposed to be examined by experiments. For this reason, Mr Newton in his Opticks distinguished those things which were made certain by experiments from those things which remained uncertain, and which he therefore proposed in the end of his Opticks in the form of queries. For this reason, in the preface to his Principles, when he had mentioned the motions of the planets, comets, moon and sea as deduced in this book from gravity, he added: ‘If only we could derive the other phenomena of nature from mechanical principles by the same kind of reasoning! For many things lead me to have a suspicion that all phenomena may depend on certain forces by which the particles of bodies, by causes not yet known, either are impelled towards one another and cohere in regular figures, or are repelled from one another and recede. Since these forces are unknown, philosophers have hitherto made trial of nature in vain.’ And in the end of this book in the second edition, he said that for want of a sufficient number of experiments, he forbore to describe the laws of the actions of the spirit or agent by which this attraction is performed. And for the same reason he is silent about the cause of gravity, there occurring no experiments or phenomena by which he might prove what was the cause thereof. And this he hath abundantly declared in his Principles, near the beginning thereof, in these words: ‘I am not now considering the physical causes and sites of forces’ [definition 8].