394 THE PRINCIPLES OF SCIENCE. 



forbidden by the nature of space. A classification is essen- 

 tially a mental not a material thing. 



Discovery of Marks or Characteristics. 



Although the chief purpose, of classification is to disclose 

 the deepest and most general resemblances of the objects 

 classified, yet the practical value of any particular system 

 will partly depend upon the ease with which we can refer 

 an object to its proper class, and thus infer concerning it 

 all that is known generally of that class. This operation 

 of discovering to which class of a system a certain speci- 

 men or case belongs is generally called Diagnosis, a 

 technical term very familiarly used by physicians, who 

 constantly require to diagnose or determine the nature 

 of the disease from which a patient is suffering. Now 

 every class is defined by certain specified qualities or cir- 

 cumstances, the whole of which are present in every object 

 contained in the class, and not all present in any object 

 excluded from it. These defining circumstances ought 

 to consist of the deepest and most important circum- 

 stances, by which we vaguely mean those probably 

 forming the conditions with which the minor circum- 

 stances are correlated. But it will often happen that the 

 so-called important points of an object are not those which 

 can most readily be observed. Thus the two great classes 

 of phanerogamous plants are defined respectively by the 

 possession of two cotyledons or seed-leaves, and one coty- 

 ledon. But when a plant comes to our notice and we 

 want to refer it to the right class, it will often happen 

 that we have no seed at all to examine, in order to dis- 

 cover whether there be one seed-leaf or two in the germ. 

 Even if we have a seed it will often be very small, and a 

 careful^dissection under the microscope will be requisite to 

 ascertain the number of cotyledons. Occasionally the 



