10 Astronomy and Geology compared. PT. i. 



method of reasoning and observation a very high 

 degree of proof may be obtained, but it is not quite 

 so overwhelmingly convincing as are the conclusions 

 drawn from mathematical demonstration. Let us 

 compare two examples of proofs obtained by each 

 of these two methods. Nothing can be much more 

 certain upon this earth than that every oak has 

 sprung originally from an acorn, and that a sackful 

 of acorns planted at the proper season and in favour- 

 able soil will produce a whole crop of young oaks ; 

 but our only ground for believing this is founded 

 upon the past experience of similar facts. We can 

 have no possible means of knowing, if shown an 

 acorn for the first time, that it would produce an 

 oak ; no reasoning of the a priori kind could lead 

 us to such a conclusion ; we know that the con- 

 sequence will be such merely from universal pre- 

 vious experience. The nature of the proof differs 

 altogether from that by which we arrive at the 

 certainty that the angles at the base of an isosceles 

 triangle are equal. In the one case the result is 

 certain under all circumstances, and arises from the 

 nature of things. In the other it is a consequence 

 which we have observed to follow under existing 

 conditions from certain premises, but we cannot 



