374 LOGIC : OR, T}E RIGHT VSE OF REASON. SECT. V. Rules of Definition of the Thing. THE special rules of a good definition are these : Rule I. !í definition must be universal, or as some call it adequate; that is, it must agree to all the particular species or individuals that are included under the same idea ; so the juice of the grape agrees to all proper wines, whether red, white, French, Spanish, Florence, &c. II. It must be proper and peculiar to the thing defined,, and agree to that alone ; for it is the very design of a definition effec- tually to distinguish one thing from all othors ; so the juice of a grape agrees to no other substance, to no other liquid, to no other being but wine. These two rules being observed, will always render a defi- nition reciprocal with the thing defined ; which is a scholastic way of speaking, to signify that the definition may be used in any sentence in the place of the thing defined ; or they may be mu- tually affirmed concerning eachother, or substituted in the room of each other. The juice of the grape is wine, or wine is the juice of the grape. And wheresoever the word wine is used, you may put the juice of the grape instead of it, except when you consider wine rather as a word than a thing, or when it is men- tioned in such logical rules. III. A definition ought to be clear and plain ; for the design of it is to lead us into the knowledge of the thing defined. Hence it Will follow, that the words used in a definition ought not to be doubtful, or equivocal, and obscure, but as plain and easy as the language will afford; and indeed it is a ge- neral rule concerning the definition of both names and things, that no word should be used in either of them, which has any darkness or difficulty in it, unless it has been before explained or defined. Hence it will follow also, that there are many things which cannot well be defined either as to the name or the thing, unless it be by synonymous words or by a negation of the contrary idea, &c. for learned men know not how to make them more evident, or more intelligible, than the ideas which every man has gained by the vulgar methods of teaching. Such are the ideas of exten- sion, duration, thought, consciousness, and most of our simple ideas, and particularly sensible qualities, as white, blue, red cold, heat, shrill, bitter, sour, &c. We can say of duration, that it is a continuance in being, or a not ceasing to be ; we can say of consciousness that it is as it 'were a feeling within ourselves we may say heat is that which is not cold ; or sour is that which is like vinegar ; or we way point to the clear sky, and say that is blue. These are the vul- gar methods of teaching the definitions of naines, or meaning Of words. But there are some philosophers, whose attempts to de-
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