VOL. XXIX.] PHILOSOPHICAL TRANSACTIONS. 151 



reason, Mr. Newton in his Optics 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 Optics 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 : " I wish the other phaenomena of nature could by the same 

 way of reasoning be deduced from mechanical principles; for, several things 

 induce me to believe, that all these things may depend upon certain forces, 

 by which the particles of bodies are, by causes still unknown to us, either mu- 

 tually impelled towards each other, and cohere together according to certain 

 regular configurations, or mutually recede from each other ; and for want of 

 knowing these forces philosophers have hitherto attempted to no purpose to 

 explain nature." And in the end of this book, in the 2d 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 phaenomena, by which he might prove what was the cause 

 of it. And this he has abundantly declared in his Principles, near the begin- 

 ning, in these words : " I do not inquire into the physical causes and seats of 

 forces." And a little after ; " I indifferently and promiscuously use for each 

 other the words attraction, impulse, or any kind of propension towards the 

 centre, by considering these forces not physically but mathematically. Whence 

 I would caution the reader not to think, that by these words I define the species 

 or manner of the action, or the physical cause or reason; or that I truly and 

 physically ascribe forces to centres, which are only mathematical points, if I 

 should happen to say, that either the centres attract, or that there are central 

 forces." And at the end of his Optics; " Here I do not inquire by what effi- 

 cient cause these qualities, viz. gravity, the magnetic and electrical forces are 

 produced. What I call attraction, may possibly be produced by impulse, or in 

 some other manner unknown to us. By attraction, I would here be understood 

 to mean only in general, a certain kind of force, whereby bodies mutually 

 tend towards each other, whatever cause that quality may be ascribed to. For 

 we must first necessarily know by phaenomena of nature, what bodies mutually 

 attract each other, and what are the laws and properties of that attraction, 

 before we can properly inquire by what efficient cause that attraction is pro- 

 duced." And a little after he mentions the same attractions as forces which 

 by phaenomena appear to have a being in nature, though their causes be not 

 yet known ; and distinguishes them from occult qualities, which are supposed 

 to flow from the specific forms of things. And in the scholium at the end of 



