188 INDUCTION. 



the conclusion has turned out erroneous. The experience, however, 

 on which the conclusion rested was genuine. From the earhest 

 records, the testimony of all the inhabitants of the known world was 

 unanimous on the point. The uniform experience, therefore, of the in- 

 habitants of the known world, agreeing in a common result, without 

 one known instance of deviation from that result, is not alwaysr suffi- 

 cient to establish a general conclusion. 



But let us now turn to an instance apparently not very dissimilar to 

 this. Mankind were wrong, it seems, in concluding that all swans 

 were white : are we also wrong, when vve conclude that all men's 

 heads grow above their shoulders, and never below, in spite of the 

 conflicting testimony of the naturalist Pliny 1 As there were black 

 swans, although civilized men had existed for three thousand years on 

 the earth without meeting with them, may there not also be " men 

 whose heads do gi'ow beneath their shoulders," notwithstanding a 

 father less perfect unanimity of negative testimony from all observers ? 

 Most persons would answer No ; it was more credible that a bird 

 should vary in its color, than that man should vary in the relative posi- 

 tion of his principal organs. And there is no doubt that in so saying 

 they would be right : but to say why they are right, would be impos- 

 sible, without entering, more deeply than is usually done, into the true 

 theory of Induction. 



Again, there are cases in which we reckon with the most unfailing 

 confidence upon uniformity, and other cases in which we do not count 

 upon it at all. In some, we feel complete assurance that the future will 

 resemble the past, the unknown be precisely similar to the known. In 

 others, however invariable may be the result obtained fi-om the in- 

 stances which we have observed, we draw from them no more than a 

 very feeble presumption that the like result will hold in all other cases. 

 That a straight line is the shortest distance between two points, we do 

 not doubt to be true even in the region of the fixed stars. When a chem- 

 ist announces the existence and properties of a newly-discovered sub- 

 stance, if we confide in his accuracy, we feel assured that the conclusions 

 he has amved at will hold universally, although the induction be founded 

 but on a single instance. We do not withhold our assent, waiting for 

 a repetition of the experiment ; or if we do, it is fi"om a doubt whether 

 the one experiment was properly made, not whether if properly made 

 it would be conclusive. Here then, is a general law of nature, in- 

 feired without hesitation fi-om a single instance ; an universal pi'opo- 

 sition from a singular one. Now mark another case, and contrast it 

 with this. Not all the instances which have been observed since the 

 beginning of the world, in support of the general proposition that all 

 crows are black, would be deemed a suffiijient presumjjtion of the 

 truth of the proposition, to outweigh the testimony of one unexcep- 

 tionable witness who should affirm that in some region of the earth 

 not fully explored, he had caught and examined a crow, and had found 

 it to be gray. 



Why is a single instance, in some cases, sufficient for a complete in- 

 duction, while in others, myriads of concurring instances, without a 

 single exception known or presumed, go such a very little way towards 

 establishing an universal proposition 1 Whoever can 'answer this ques- 

 tion knows more of the philosophy of logic than the wisest of the an- 

 cients, and has solved the great problem of induction. 



