xxiv.] EMPIRICAL KNOWLEDGE, EXPLANATION, &c. 533 



material similes. The action is ex plicis plana reddere, 

 to take out the folds, and render a thing plain or even. 

 Explanation thus renders a thing clearly comprehensible 

 in all its points, so that there is nothing left outstanding 

 or obscure. 



Every act of explanation consists in pointing out a 

 resemblance between facts, or in showing that similarity 

 exists between apparently diverse phenomena. This simi- 

 larity may be of any extent and depth ; it may be a 

 general law of nature, which harmonises the motions of 

 all the heavenly bodies by showing that there is a similar 

 force which governs all those motions, or the explanation 

 may involve nothing more than a single identity, as when 

 we explain the appearance of shooting stars by showing 

 that they are identical with portions of a comet. Wherever 

 we detect resemblance, there is a more or less explanation. 

 The mind is disquieted when it meets a novel pheno- 

 menon, one which is sui generis; it seeks at once for 

 parallels which may be found in the memory of past 

 sensations. The so-called sulphurous smell which attends 

 a stroke of lightning often excited attention, and it was 

 not explained until the exact similarity of the smell 

 to that of ozone was pointed out. The marks upon a 

 flagstone are explained when they are shown to correspond 

 with the feet of an extinct animal, whose bones are else- 

 where found. Explanation, in fact, generally commences 

 by the discovery of some simple resemblance ; the theory 

 of the rainbow began as soon as Antonio de Dominis 

 pointed out the resemblance between its colours and those 

 presented by a ray of sunlight passing through a glass 

 globe full of water. 



The nature and limits of explanation can only be fully 

 considered, after we have entered upon the subjects of 

 generalisation and analogy. It must suffice to remark, in 

 this place, that the most important process of explanation 

 consists in showing that an observed fact is one case of a 

 general law or tendency. Iron is always found combined 

 with sulphur, when it is in contact with coal, whereas in 

 other parts of the carboniferous strata it always occurs as 

 a carbonate. We explain this empirical fact as being due 

 to the reducing power of carbon and hydrogen, which pre- 

 vents the iron from combining with oxygen, and leaves it 



