LOGIC OF PRACTICE OR ART. 617 



common observation or speedily learnt from practice, 

 rules may be safely acted upon by persons who know 

 nothing more than the rule. But in the complicated 

 affairs of life, and still more in those of states and 

 societies, rules cannot be relied on, without constantly 

 referring back to the scientific laws on which they are 

 founded. To know what are the practical contin- 

 gencies which require a modification of the rule, or 

 which are altogether exceptions to it, is to know what 

 combinations of circumstances would interfere with, 

 or entirely counteract, the consequences of those laws : 

 and this can only be learnt by a reference to the theo- 

 retical grounds of the rule. 



By a wise practitioner, therefore, rules of conduct 

 will only be considered as provisional. Being made 

 for the most numerous cases, or for those of most 

 ordinary occurrence, they point out the manner in 

 which it will be least perilous to act, where time or 

 means do not exist for analyzing the actual circum- 

 stances of the case, or where for any reason we cannot 

 trust our judgment in estimating them. But they do 

 not at all supersede the propriety of going through 

 (when circumstances permit) the scientific process 

 requisite for framing a rule from the data of the par- 

 ticular case before us. At the same time, the 

 common rule may very properly serve as an admoni- 

 tion, that a certain mode of action has been found by 

 ourselves and others to succeed in the cases of most 

 common occurrence; so that if it be unsuitable in the 

 case in hand, the reason of its being so will be likely 

 to arise from some unusual circumstance. 



$ 4. The error is therefore apparent, of those 

 who would deduce the line of conduct proper to par- 

 ticular cases, from supposed universal practical 



