_We collect individuals into_ Kinds _by applying to them the Idea of Likeness. Kinds of things are not determined by definitions, but by this condition:--that general assertions concerning such kinds of things shall be possible._ (VIII. 1.) LXXXIV. _The_ Names _of kinds of things are governed by their use; and that may be a right name in one use which is not so in another. A whale is not a_ fish _in natural history, but it is a_ fish _in commerce and law._ (VIII. 1.) LXXXV. _We take for granted that each kind of things has a special_ character _which may be expressed by a Definition. The ground of our assumption is this;--that reasoning must be possible._ (VIII. 1.) LXXXVI. _The "Five Words,"_ Genus, Species, Difference, Property, Accident, _were used by the Aristotelians, in order to express the subordination of Kinds, and to describe the nature of Definitions and Propositions. In modern times, these technical expressions have been more referred to by Natural Historians than by Metaphysicians._ (VIII. 1.) LXXXVII. _The construction of a Classificatory Science includes_ Terminology, _the formation of a descriptive language;_--Diataxis, _the Plan of the System of Classification, called_ {21} _also the_ Systematick;--Diagnosis, _the Scheme of the Characters by which the different Classes are known, called also the_ Characteristick. Physiography _is the knowledge which the System is employed to convey. Diataxis includes_ Nomenclature. (VIII. 2.) LXXXVIII. Terminology _must be conventional, precise, constant; copious in words, and minute in distinctions, according to the needs of the science. The student must understand the terms,_ directly _according to the convention, not through the medium of explanation or comparison._ (VIII. 2.) LXXXIX. _The_ Diataxis,_ or Plan of the System, may aim at a Natural or at an Artificial System. But no classes can be absolutely artificial, for if they were, no assertions could be made concerning them._ (VIII. 2.) XC. _An_ Artificial System _is one in which the_ smaller _groups_ (_the Genera_) _are_ natural; _and in which the_ wider _divisions_ (_Classes, Orders_) _are constructed by the_ peremptory _application of selected Characters;_ (_selected, however, so as not to break up the smaller groups._) (VIII. 2.) XCI. _A_ Natural System _is one which attempts to make_ all _the divisions_ natural, _the widest as well as the narrowest; and therefore applies_ no _characters_ peremptorily. (VIII. 2.) XCII. _Natural Groups are best described, not by any Definition which marks their boundaries, but by a_ Type _which marks their center. The Type of any natural group is an example which possesses in a marked degree all the leading characters of the class._ (VIII. 2.) {22} XCIII.