ON THE DOCTRINE OF NATURAL KINDS.

By M. H. TOWKY. During a long study of Taxonomy, it has repeatedly seemed to me that some obscurity and indefiniteness, if not error, hangs around Mill's doctrine of Natural Kinds, and it is rather to draw forth the views of others than to gain expression for my own ideas that I take up the subject in this paper. I fancy most logicians will agree with me that the keystone of the process of classifying is the doctrine of extension and inten- sion ; that the fundamentum divisionis is a purely subjective con- ception; that Discrimination, Abstraction and Generalisation are our working tools in construction ; that the whole purport of classifying is the mental methodisation of our knowledge of indi- viduals. Could the mind grasp and retain a full presentation of each Thing, instead of thinking in sequence, the contrivance would be needless. Further, numerical enumerations are the only classifications that proceed on extension alone ; arrangements based on pure intension would be equally unfruitful of results. The only legiti- mate classifications work on extension, proceeding by intension. Now, it appears to me that Mill's doctrine of Natural Kinds controverts the whole procedure and modus operandi of logical classifying, rests on an arbitrary and untenable proposition, and stands in his theory, like the pillar of Eoslin chapel, irreconcil- able with the rest of the structure. Moreover, it is alien to the actualities obtaining, so that on both counts it surely should be exa- mined, and either be remodelled or replaced by a truer doctrine. To make out my indictment it will be needful first briefly to glance at uncontroverted points of theory, in order to demonstrate what an alien stumbling-block the doctrine introduces. The Predicables were originally not a formal scheme of Classifi- cation, but an outline of the co-relations of General Terms. The basis selected by logicians on which to found an exhaustive com- prehension of General Terms was that of the relation between the subject and the predicate. Of the two general terms which form the subject and predicate of any proposition, one, they said, might stand to the other in any one of five relations. Hence it followed that, as these distinc- tions were entirely relative, some general terms might be, in different sentences, either genus or species, and others either differentia, property or accident. In a short time this meaning of the Predicables was laid aside, and the names were used in the formal analysis of classifying which laid down the requisites of correct diataxis and subdivision. The Predicables were applied not to the general terms, but to the divisions and attributes themselves, and this meaning has so largely displaced the former