16 PRINCIPLES OF SCIENCE, 



not be proved by the nature of brutal instine^, 

 any more than the vitality of plants can be 

 proved from the chemical properties of matter, 

 dead and common: in this case, physiology 

 and physics, chemistry and metaphysics, would 

 be confounded together. 



The same observations may be matfe, if the 

 facts which appertain to vision, were employed 

 to account for the cause of hearing, or of those 

 which appertain to taste, with those which be- 

 long to the olfactory sense. 



ilf we proceed from physiological to mecha- 

 nical sciences, the same remarks will be equally 

 applicable. If the principles of hydraulics 

 were employed to account for the effects pro- 

 duced in pneumatics ; and, even if those of 

 pneumatics were advanced to prove the nature 

 of optics, such facts would be false ; as false, 

 as if we were to confound the facts which apper- 

 tain to time, with those that belong to place ; 

 figures with lines, and lines with figures ; and, 

 attempt to prove magnitude by numbers, or 

 numbers by magnitude; and confound geometry 

 and arithmetic together. For the express pur- 

 pose of guarding against this great error, it 

 would seem, that Sir ISAAC NEWTON, in his uni- 

 versal arithmetic, praises the antients for not 

 deducing geometrical conclusions from arith- 

 metical principles, and for not confounding geo- 

 metry and arithmetic together. " Each of these 



