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INDUCTION. 



ought to be grounded on an examination of all the infimce 

 species comprehended in them, and not of a portion only. We 

 cannot conclude (where causation is not concerned), because a 

 proposition is true of a number of things resembling one 

 another only in being animals, that it is therefore true of all 

 animals. If, indeed, anything be true of species which differ 

 more from one another than either differs from a third, 

 (especially if that third species occupies in most of its known 

 properties a position between the two former,) there is some 

 probability that the same thing will also be true of that inter- 

 mediate species ; for it is often, though by no means univer- 

 sally, found, that there is a sort of parallelism in the properties 

 of different Kinds, and that their degree of unlikeness in 

 one respect bears some proportion to their unlikeness in 

 others. We see this parallelism in the properties of the 

 different metals; in those of sulphur, phosphorus, and carbon; 

 of chlorine, iodine, and bromine ; in the natural orders of plants 

 and animals, &c. But there are innumerable anomalies and 

 exceptions to this sort of conformity; if indeed the con- 

 formity itself be anything but an anomaly and an exception in 

 nature. 



Universal propositions, therefore, respecting the properties 

 of superior Kinds, unless grounded on proved or presumed 

 connexion by causation, ought not to be hazarded except after 

 separately examining every known sub-kind included in the 

 larger Kind. And even then such generalizations must be 

 held in readiness to be given up on the occurrence of some 

 new anomaly, which, when the uniformity is not derived from 

 causation, can never, even in the case of the most general of 

 these empirical laws, be considered very improbable. Thus 

 all the universal propositions which it has been* attempted to 

 lay down respecting simple substances, or concerning any of 

 the classes which have been formed among simple substances, 

 (and the attempt has been often made,) have, with the pro- 

 gress of experience, either faded into inanity, or been proved 

 to be erroneous ; and each Kind of simple substance remains 

 with its own collection of properties apart from the rest, saving 

 a certain parallelism with a few other Kinds, the most similar 



