LOGIC OF PRACTICE OB ART. 547 



cumstances which the science exhibits as conditions, are 

 prescribed as means. 



It is true that, for the sake of convenience, rules must be 

 formed from something less than this ideally perfect theory ; 

 in the first place, because the theory can seldom be made 

 ideally perfect; and next, because, if all the counteracting 

 contingencies, whether of frequent or of rare occurrence, were 

 included, the rules would be too cumbrous to be apprehended 

 and remembered by ordinary capacities, on the common occa- 

 sions of life. The rules of art do not attempt to comprise 

 more conditions than require to be attended to in ordinary 

 cases; and are therefore always imperfect. In the manual 

 arts, where the requisite conditions are not numerous, and 

 where those which the rules do not specify are generally either 

 plain to common observation or speedily learnt from practice, 

 rules may often be safely acted on 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 contingencies 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 theoretic 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 analysing the actual 

 circumstances of the case, or where we cannot trust our 

 judgment in estimating them. But they do not at all super- 

 sede the propriety of going through (when circumstances 

 permit) the scientific process requisite for framing a rule from 

 the data of the particular case before us. At the same time, 

 the common rule may very properly serve as an admonition 



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