536 DISSERTATION ON 



accuflomed to the accuracy and propriety of the language of antleat 

 philolbphy, will be furprifed to find that the principle of reli and niac- 

 tivity of body (hould be called by the name of 'uis, and denoted, 

 botli by the term vis inftta^ and the term vis incrtuie^ nomine 

 ftgnifcantiffifno^ as Sir Ifaac fays in his expUnat»on of his third defini- 

 tion ; and he will be more lurpriled ihll to find the fame term of vis 

 inertiae applied to the principle by which body continues in motion ; 

 and which, therefore, according to the. common ufe of words, fhould 

 be rather called vis mobilitatis. 



But, allowing the Newtonians to fpeakas inaccurately as they pleafe, 

 though there be fome realon to think that inaccuracy of expreflioa 

 has led many of them to inaccuracy of thinking, let us come more 

 clofely to the queftion. — And, in the jirjl place, it muft be ad- 

 mitted, in point of fad, that a body here on earth, after it is once put 

 in motion by bodily impulfe, continues, at leaft, for fome time, in mo- 

 tion ; and, if this motion be efTential to matter, it is no doubt true, as 

 I have faid, that body moves itielf, and that the motion muft continue 

 forever, or, at leaft, as long as the body exifts. The queftion, there- 

 fore, is concerning the nature and eflence of matter, or body, which, 

 in this inquiry, I confider as the fame thing ; and fo I fee it is confi- 

 dcrcd by 8ir Ifaac, who ufes the word materia and corpus incJifterentiy. 

 Now, though it be impoflible to fay what is the fubftance or efllnce 

 of matter, fince, as I have had occafion, more than once, to oblerve, 

 we know not, in this ftate of our exiftence, the eflence of any one na- 

 tural thing, there is an eflential quality of matter, which we know, 

 fo eflential, that, according to the philofophy of antiquity, it is pre- 

 vious to all its other qualities ; the quality I mean is pajfivity and 

 perfe^ ina^ivitj, which makes [it a fit fubjedt to receive all forms, 

 qualities, and properties, of every kind ; and therefore is held by 

 thofe philofophers to be, in the order of nature, prior to them all, 

 even to fuch as we reckon moft efl^ential to matter, luch as iolidity, 



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