CORRESPONDENCE, 1830-40. 147 



* Has he examined all the particular cases of his proposition ? ' 1842. 

 He either knows this a priori or he has examined (that is, if he 

 knows it to be true) ; if he knows it a priori, then he opens the 

 net, and out we jump; but I will answer for it he has not 

 examined all the possible cases. 



Is there not a confused way of talking about truth ? When 

 we prove a truth, that is, give ourselves certain knowledge of its 

 being a truth, we talk as if we had made a truth. We say ' one 

 proposition is the consequence of another ; ' when it should be 

 ' our knowledge of one proposition is the consequence of our 

 knowledge of another.' 



He who said peace among men forbade Metaphysics. When 

 their cloudinesses the axioms of mental philosophy declare war 

 against ' two straight lines can't enclose a space ' they remind me 

 of the Chinese trying to take an English battery. 



Here are some undoubted truths : 



sin oo =0, cos co =0 ; 



tan oo ^^v" 1, cot co = + / 1. 



As to sec oo and cosec oo I am doubtful. They are either 

 or oo . I suspect the former. 



Yours very truly, 



A. DE MORGAN. 



To Sir John Herschel. 



69 Gower Street, Dec. 30, 1842. 

 MY DEAR SIR JOHN, Many thanks for the reduction of 



Schiller's observations to the latitude of London. I dare sav 



j 



you have applied the correction 



-f (English German) 



very skilfully, but I am so ignorant of that language that I shall 

 not find you out if you were to err in the first place of gutturals. 



When your missive arrived, I was engaged with a young 

 Turk whom I indoctrinate in differential equations and matters 

 arising thereout. I gave him your wafer as a sort of auto- 

 graph, whereupon the following dialogue took place : 



He. Oh, Sir John Herschel ! what is it he has done with the 



L 2 



