rhe P R E E A C E. xiii 
fundamental unerring Rule in 
“ Phyficks, TTiat it is not ’within 
“ the compafs of human Underjiand-^ 
“ ing to ajftgn a purely fpeculative 
Reafon for any one Phcenomenon 
“ in Nature!^ So that in natural 
Philofophy, we cannot depend on 
any meer Speculations of the Mind ; 
we can only with the Mathemati- 
cians, reafon with any tolerable Cer- 
tainty from proper Data^ fuch as 
arife from the united Teftimony 
of many good and credible Expe- 
riments. 
Yet it feems not unreafonablc on 
the other hand, tho’ not far to in- 
dulge, yet to carry our Reafonings 
a little farther than the plain Evi- 
dence of Experiments will warrant; 
for fince at the utmoft Boundaries 
of thofe Things which we clearly 
know, there is a kind of Twilight 
call 
