INTRODUCTION. 17 



this relation, (which, indeed, is one of the hardest 

 problems of our philosophy,) we may observe that 

 they have this in common, that both are acquired 

 by acts of the mind exercised in connecting ex- 

 ternal impressions, and may be employed in con- 

 ducting a train of reasoning ; or, speaking loosely, 

 (for we cannot here pursue the subject so as to 

 arrive at philosophical exactness,) we may say, that 

 all notions and ideas are obtained by an inductive, 

 and may be used in a deductive process. But sci- 

 entific Ideas and common Notions differ in this, that 

 the former are precise and stable, the latter vague 

 and variable; the former are possessed with clear 

 insight, and employed in a sense rigorously limited, 

 and always identically the same; the latter have 

 grown up in the mind from a thousand dim and 

 diverse suggestions, and the obscurity and incon- 

 gruity which belongs to their origin hangs about 

 all their applications. Scientific Ideas can often be 

 adequately exhibited for all the purposes of rea- 

 soning, by means of Definitions and Axioms; all 

 attempts to reason by means of Definitions from 

 common Notions, lead to empty forms or entire con- 

 fusion. 



Such common Notions are sufficient for the com- 

 mon practical conduct of human life; but man is 

 not a practical creature merely ; he has within him 

 a speculative tendency, a pleasure in the contem- 

 plation of ideal relations, a love of knowledge as 

 knowledge. It is this speculative tendency which 

 VOL. i. C 



