DEFINITION. 109 



it with some other things. It is only necessary that the definition (or de- 

 scription) thus formed, should be convertible with the name which it pro- 

 fesses to define ; that is, should be exactly co-extensive with it, being pred- 

 icable of every thing of which it is predicable, and of nothing of which it 

 is not predicable ; though the attributes specified may have no connection 

 with those which mankind had in view when they formed or recognized the 

 class, and gave it a name. The following are correct definitions of Man, 

 according to this test : Man is a mammiferous animal, having (by nature) 

 two hands (for the human species answers to this description, and no other 

 animal does) : Man is an animal who cooks his food : Man is a featherless 

 biped. 



What would otherwise be a mere description, may be raised to the rank 

 of a real definition by the peculiar purpose which the speaker or writer 

 has in view. As was seen in the preceding chapter, it may, for the ends 

 of a particular art or science, or for the more convenient statement of an 

 author's particular doctrines, be advisable to give to some general name, 

 without altering its denotation, a special connotation, different from its or- 

 dinary one. When this is done, a definition of the name by means of the 

 attributes which make up the special connotation, though in general a mere 

 accidental definition or description, becomes on the particular occasion and 

 for the particular purpose a complete and genuine definition. This actual- 

 ly occurs with respect to one of the preceding examples, " Man is a mam- 

 miferous animal having two hands," which is the scientific definition of 

 man, considered as one of the sj^ecies in Cuvier's distribution of the animal 

 kingdom. 



In cases of this sort, though the definition is still a declaration of the 

 meaning which in the particular instance the name is appointed to convey, 

 it can not be said that to state the meaning of the word is the purpose of 

 the definition. The purpose is not to expound a name, but a classification. 

 The special meaning which Cuvier assigned to the word Man (quite foreign 

 to its ordinary meaning, though involving no change in the denotation of 

 the word), was incidental to a plan of arranging animals into classes on a 

 certain principle, that is, according to a certain set of distinctions. And 

 since the definition of Man according to the ordinary connotation of the 

 word, though it would have answered every other purpose of a definition, 

 would not have pointed out the place which the species ought to occupy 

 in that particular classification ; he gave the word a special connotation, 

 that he might be able to define it by the kind of attributes on which, for 

 reasons of scientific convenience, he had resolved to found his division of 

 animated nature. 



Scientific definitions, whether they are definitions of scientific terms, or 

 of common terms used in a scientific sense, are almost always of the kind 

 last spoken of : their main purpose is to serve as the landmarks of scien- 

 tific classification. And since the classifications in any science are con- 

 tinually modified as scientific knowledge advances, the definitions in the 

 sciences are also constantly varying. A striking instance is afforded by 

 the words Acid and Alkali, especially the former. As experimental dis- 

 covery advanced, the substances classed with acids have been constantly 

 multiplying, and by a natural consequence the attributes connoted by the 

 word have receded and become fewer, At first it connoted the attributes, 

 of combining with an alkali to form a neutral substance (called a salt) ; 

 being compounded of a base and oxygen ; causticity to the taste and touch ; 

 fluidity, etc. The true analysis of muriatic acid, into chlorine and hydro- 



