228 INDUCTION. 



ine. From the earliest records, the testimony of the inhabitants of the 

 known world was unanimous on the point, ^he uniform experience, there- 

 fore, of the inhabitants of the known worlaj^agreeing in a common result, 

 without one known instance of deviation from that result, is not always 

 sufficient 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 we conclude that all men's heads grow above their 

 shoulders, and never below, in spite of the conflicting testimony of the natu- 

 ralist Pliny ? As there were black swans, though civilized people had exist- 

 ed for thi'ee thousand years on the earth without meeting with them, may 

 there not also be " men whose heads do grow beneath their shoulders," not- 

 withstanding a rather less perfect unanimity of negative testimony from 

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

 bird should vary in its color, than that men should vary in the relative po- 

 sition of their principal organs. And there is no doubt that in so saying 

 they would be right : but to say why they are right, would be impossible, 

 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 confi- 

 dence 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 from the instances which have been 

 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 short- 

 est distance between two points, we do not doubt to be true even in the re- 

 gion of the fixed stars.* When a chemist announces the existence and 

 properties of a newly-discovered substance, if we confide in his accuracy, 

 we feel assured that the conclusions he has arrived at will hold universally, 

 though 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 from 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, inferred without hesitation from a single instance ; a uni- 

 versal proposition from a singular one. Now mark another case, and con- 

 trast 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 sufficient presumption of the truth of 

 the proposition, to outweigh the testimony of one unexceptionable 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 induc- 

 tion, while in others, myriads of concurring instances, without a single ex- 

 ception known or presumed, go such a very little way toward establishing 

 a universal proposition ? Whoever can answer this question knows more 

 of the philosophy of logic than the wisest of the ancients, and has solved 

 the problem of induction. 



* In strictness, wherever the present constitution of space exists ; which we have ample 

 reason to believe that it does in the region of the fixed stars. 



