INTRODUCTION. 15 



Before we proceed to the examination of its several parts, we must pause to 

 consider the mode of reasoning which is the most generally to be adopted. 

 It depends on the axiom which has always been essentially concerned in every 

 improvement of natural philosophy, but which has been more and more em- 

 ployed, ever since the revival of letters, under the name induction, and which 

 has been sufficiently discussed by modern metaphysicians. That like causes 

 produce like effects, or, that in similar circumstances similar consequences en- 

 sue, is the most general and most important law of nature; it is the founda- 

 tion of all analogical reasoning, and is collected from constant experience, by 

 an indispensable and unavoidable propensity of the human mind. 



It does not appear that we can have any other accurate conception of cau- 

 sation, or of the connexion of a cause with its effect, than a strong impression 

 of the observation, from uniform experience, that the one has constantly fol- 

 lowed the other. We do not know the intimate nature of the connexion by 

 which gravity causes a stone to fall, or how the string of a bow urges the 

 arrow forwards ; nor is there any original absurdity in supposing it possible 

 that the stone might have remained suspended in the air, or that the bow- 

 string might have passed through the arrow as light passes through glass. 

 But it is obvious that we cannot help concluding the stone's, weight to be the 

 cause of its fall, and that every heavy body will fall unless supported; and 

 the pressure of the string to be the cause of the arrow's motion, and, that if 

 we shoot, the arrow will fly ; if we hesitated to make these conclusions, "we 

 should often pay de:ir for our scepticism. This explanation is sufficient to 

 show the identity of the two expressions, that like causes produce like effects, 

 and, that in similar circumscances similar consequences ensue. And such is 

 the ground of argument from experience, the simplest principle of reasoning, 

 after pure nK! them atical truths; which appear to be so far prior to experi- 

 ence, as their contradiction always implies an absurdity repugnant to the 

 imaginatioco 



In the application of induction, the greatest caution and circmnspection 

 are necessary ; for it is obvious that, before we can infer with certainty the 

 complete similarity of two ^ents, we must be perfectly well assured that we 

 are acquainted with every circumstance which can have any relation to their 

 causes. The error of some of the ancient schools consisted principally in the 



