CLA SSI PICA TION. 397 



matter of the greatest importance, for, unless we can 

 recognise, from time to time, objects or substances which 

 have been before investigated, all recorded discoveries 

 would lose their value. Even a single investigator must 

 have some means of recording or systematizing his ob- 

 servations of any large number of objects like those 

 furnished by the vegetable and animal kingdoms. 



Now whenever a class has been properly formed, a 

 definition must have been laid down, stating the qualities 

 and circumstances possessed by all the objects which are 

 intended to be included in the class, and not possessed 

 completely by any other objects. Diagnosis, therefore, 

 consists simply in comparing the qualities of a certain 

 object with the definitions of a series of classes ; the 

 absence in the object of any one quality stated in the 

 definition excludes it from the class thus denned ; whereas, 

 if we find every point of a definition exactly fulfilled in 

 the specimen, we may at once assign it to the class in 

 question. It is of course by no means certain that every- 

 thing which has been affirmed of a class is true of all 

 objects afterwards referred to the class ; for this would 



(be a case of imperfect inference, which is never more 

 than a matter of probability. A definition can only make 

 known a finite number of the properties of an object, so 

 that it always remains possible that objects agreeing in 

 those assigned properties will differ in other ones. An 

 individual cannot be defined, and can only be made known 

 by the exhibition of the individual itself, or by a material 

 specimen exactly representing it. But this and many 

 other questions relating to definition must be treated if 

 I am able to take up the general subject of language in 

 another work. 



Diagnostic systems of classification should, as a general 

 rule, be arranged on the bifurcate method explicitly. Any 

 property may be chosen which divides the whole group 



