INDUCTION. 165 



data afforded by the objects which have been examined. 

 If I judge that a distant star obeys the law of gravity, 

 it must be because all other material objects sufficiently 

 known to me obey that law. If I venture to assert that 

 all ruminant animals have cloven hoofs, it is because all 

 ruminant animals which have come to my notice have 

 cloven hoofs. On the other hand I cannot safely say 

 that all cryptogamous plants possess a purely cellular 

 structure, because some such plants have -a partially 

 vascular structure. The probability that a new crypto- 

 gam will be cellular only can be estimated, if at all, on the 

 ground of the comparative numbers of known cryptogams 

 which are and are not cellular. Thus the first step in 

 every induction will consist in accurately summing up 

 the number of instances of a particular object or pheno- 

 menon which have fallen under our observation. Adams 

 and Leverrier, for instance, must have inferred that the 

 undiscovered planet Neptune would obey Bode's law, 

 because all the planets known at that time obeyed it. On 

 what principles and on what circumstances the passage 

 from the known to the apparently unknown is warranted, 

 must be carefully discussed in the next section, and in 

 various parts of this work. 



It would be a great mistake, however, to suppose that 

 Perfect Induction is in itself useless. Even when the 

 enumeration of objects belonging to any class is complete, 

 and admits of no inference to unexamined objects, the 

 enumeration of our knowledge in a general proposition is 

 a process of so much importance that we may consider it 

 practically necessary. In many cases we may render our 

 investigations exhaustive ; all the teeth or bones of an 

 animal ; all the cells in a minute vegetable organ ; all the 

 caves in a mountain side ; all the strata in a geological 

 section ; all the coins in a newly found hoard, may be so 

 completely scrutinized that we may make some general 



