CHAPTER III. 



PROPOSITIONS. 



WE now proceed to consider the variety of forms of pro- 

 positions in which the truths of science must be expressed. 

 I shall endeavour to show that, however diverse these 

 forms may be, they all admit the application of the one 

 same principle of inference that what is true of a thing is 

 true of the like or same. This principle holds true what- 

 ever be the kind or manner of the likeness, provided 

 proper regard be had to its nature. Propositions may 

 assert an identity of time, space, manner, quantity, degree, 

 or any other circumstance in which things may agree or 

 differ. 



We find an instance of a proposition concerning time in 

 the following : " The year in which Newton was born, 

 was the year in which Galileo died." This proposition 

 expresses an approximate identity of time between two 

 events; hence whatever is true of the year in which 

 Galileo died is true of that in which Newton was born, 

 and vice versd. " Tower Hill is the place where Raleigh 

 was executed " expresses an identity of place ; and what- 

 ever is true of the one spot is true of the spot otherwise 

 defined, but in reality the same. In ordinary language we 

 have many propositions obscurely expressing identities 

 of number, quantity, or degree. " So many men, so many 

 minds," is a proposition concerning number, that is to say, 

 an equation ; whatever is true of the number of men is 

 true of the number of -minds, and vice versd. " The density 

 of Mars is (nearly) the same as that of the Earth," " The 

 force of gravity is directly as the product of the masses, and 



