F R E F A C E* 
BOUT twelve years ago, not long after the author of this effaj 
* ^ had left the fchools of medicine, he began to be diffatisfied with 
the common theories of refpiration and the heart’s motion ; and as he 
had not met with any writer, who had given, as he thought, a juft ac¬ 
count of the vital and other involuntary motions of animals, or derived 
them rightly from their true fource, he purpofed fome time or other to 
write on this fubjed, if not for the public,, at leaft f^; his own fatisfac- 
tion. In purfuance of this refolution, the following effay was begun in 
« - 
the year 1744, and might have been finifhed long ago, had not the au^- 
thor’s time been greatly taken up with more neceffary buftnefs. In com- 
poling it, he has been careful not to indulge his fancy, in wantonly 
framing hypothefes, but has rather endeavoured to- proceed upon the 
furer foundations of experiment and obfervation. No doctrine in phi- 
lofophy, which was not built on thefe, has ever been able to ftand its? 
ground for half a century;, and the theories of Newton, and fome few 
others of the more happy philofophers, have therefore triumphed over 
all objedions, becaufe they were founded on nothing elfe but plain 
fads ; fads indeed, whofe exiftence was perhaps unknown before, and 
whofe influence is fo extenfive, that while they are fimple and uniform 
in themfelves, they ferve as caufes for explaining innumerable effeds. 
On the other hand, in the hypothetical method of philofophifing, caufes 
are ufually affigned, which not only cannot be proved to exift, but which 
are frequently more intricate and complex than even the effeds to be ex¬ 
plained from them. And indeed, it cannot be expeded that unguided 
imagination