INTRODUCTION. 49 



notions with which men are conversant in the common course of 

 practical life, which give meaning to their familiar language, and 

 employment to their hourly thoughts, are compared with the Ideas on 

 which exact science is founded, we find that the two classes of intel- 

 lectual operations have much that is common and much that is dif- 

 ferent. Without here attempting fully to explain this relation (whicli, 

 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 external impressions, and 

 may be employed in conducting 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 scientific 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 lim- 

 ited, and always identically the same ; the latter have grown up in the 

 mind from a thousand dim and diverse suggestions, and the obscurity 

 and incongruity" which belong to their origin hang about all their 

 applications. Scientific Ideas can often be adequately exhibited for 

 all the purposes of reasoning, by means of Definitions and Axioms ; 

 all attempts to reason by means of Definitions from common Notions, 

 lead to empty forms or entire confusion. 



Such common Notions are sufficient for the common practical con 

 duct of human life : but man is not a practical creature merely ; he 

 has within him a speculative tendency, a pleasure in the contemplation 

 of ideal relations, a love of knowledge as knowledge. It is this 

 speculative tendency which brings to light the difference of common 

 Notions and scientific Ideas, of which we have spoken. The mind 

 analyzes such Notions, reasons upon them, combines and connect? 

 them ; for it feels assured that intellectual things ought to be able to 

 bear such handling. Even practical knowledge, we see clearly, is not 

 possible without the use of the reason ; and the speculative reason is 

 only the reason satisfying itself of its own consistency. The specula- 

 tive faculty cannot be controlled from acting. The mind cannot but 

 claim a right to speculate concerning all its own acts and creations : 

 yet, when it exercises this right upon its common practical notion?, 

 we find that it runs into barren abstractions and ever-recurring cycles 

 of subtlety. Such Notions are like waters naturally- stagnant ; how- 

 ever much we urge and agitate them, they only revolve in stationan 

 VOL. I. 1 



