OF ARTS AND SCIENCES : APRIL 9, 1867. 261 



How should they treat a consumptive Englishman? Mr. Venn has 

 made an error in answering the question, but the illustration puts in a 

 clear light the advantage of ceasing to speak of probability, and of 

 speaking only of the relative frequency of this event to that* 



Five hundred and eighty-first Meeting. 



April 9, 1867. — Monthly Meeting. 



The President in the chair. 



The following paper was presented. 



On the Natural Classification of Arguments. By C. S. Peirce. 



Part I. § 1. Essential Parts of an Argument. 



In this paper, the term " argument " will denote a body of premises 

 considered as such. The term " premise " will refer exclusively to 

 something laid down, (whether in any enduring and communicable 

 form of expression, or only in some imagined sign,) and not to any- 

 thing only virtually contained in what is said or thought, and also ex- 

 clusively to that part of what is laid down which is (or is supposed to 

 be) relevant to the conclusion. 



Every inference involves the judgment that, if such propositions as 

 the premises are are true, then a proposition related to them, as the 

 conclusion is, must be, or is likely to be, true. The principle implied 

 in this judgment, respecting a genus of argument, is termed the leading 

 principle of the argument. 



A valid argument is one whose leading principle is true. 



In order that an argument should determine the necessary or proba- 

 ble truth of its conclusion, both the premises and leading principle 

 must be true. 



§ 2. Relations between the Premises and Leading Principle. 



The leading principle contains, by definition, whatever is considered 

 requisite besides the premises to determine the necessary or probable 

 truth of the conclusion. And as it does not contain in itself the 

 subsumption of anything under it, each premise must, in fact, be 

 equivalent to a subsumption under the leading principle. 



* See a notice, Venn's Logic of Chance, in the North American Review for July, 

 1867. 



