426 ■ INDUCTION. 



vation we assure ourselves of its existence ; when it is beyond tlmt range, 

 and is therefore said to be absent, we infer its existence from marks or evi- 

 dences. But what can these evidences be? Other phenomena ; ascertain- 

 ed by induction to be connected with the given phenomenon, either in the 

 way of succession or of co-existence. The simple existence, therefore, of an 

 individual phenomenon, when not directly perceived, is inferred from some 

 inductive law of succession or co-existence ; and is consequently not amen- 

 able to any peculiar inductive principles. We prove the existence of a 

 thing, by proving that it is connected by succession or co-existence with 

 some known thing. 



With respect to general propositions of this class, that is, which affirm the 

 bare fact of existence, they have a peculiarity which renders the logical 

 treatment of them a very easy matter ; they are generalizations which are 

 sufficiently proved by a single instance. That ghosts, or unicorns, or sea- 

 serpents exist, would be fully established if it could be ascertained posi- 

 tively that such things had been even once seen. Whatever has once hap- 

 pened, is capable of happening again ; the only question relates to the con- 

 ditions under which it happens. 



So far, therefore, as relates to simple existence, the Inductive Logic has 

 no knots to untie. And we may proceed to the remaining two of the great 

 classes into which facts have been divided; Resemblance, and Order in 

 Place. 



§ 2. Resemblance and its opposite, except in the case in which they as- 

 sume the names of Equality and Inequality, are seldom regarded as sub- 

 jects of science; they are supposed to be perceived by simple apprehen- 

 sion ; by merely applying our senses or directing our attention to the two 

 objects at once, or in immediate succession. And this simultaneous, or 

 virtually simultaneous, application of our faculties to the two things which 

 are to be compared, does necessarily constitute the ultimate appeal, wher- 

 ever such application is practicable. But, in most cases, it is not practica- 

 ble : the objects can not be brought so close together that the feeling of 

 their resemblance (at least a complete feeling of it) directly arises in the 

 mind. We can only compare each of them with some thii'd object, capa- 

 ble of being transported from one to the other. And besides, even when 

 the objects can be brought into immediate juxtaposition, their resemblance 

 or difference is but imperfectly known to lis, unless we have compared 

 them minutely, part by part. Until this has been done, things in reality 

 very dissimilar often appear undistinguishably alike. Two lines of very 

 unequal length will appear about equal when lying in different directions ; 

 but place them parallel with their farther extremities even, and if we look 

 at the nearer extremities, their inequality becomes a matter of direct per- 

 ception. 



To ascertain whether, and in what, two phenomena resemble or differ, is 

 not always, therefore, so easy a thing as it might at first appear. When 

 the two can not be brought into juxtaposition, or not so that the observer 

 is able to compare their several parts in detail, he must employ the indi- 

 rect means of reasoning and general propositions. When we can not bring 

 two straight lines together, to determine whether they are equal, we do it 

 by the physical aid of a foot-rule applied first to one and then to the other, 

 and the logical aid of the general proposition or formula, "Things which 

 are equal to the same thing are equal to one another." The comparison 

 of two things through the intervention of a third thing, when their direct 



