A _genus_ may be defined as a class of things the name of which recalls _fewer_ particulars than the name of another class or species with which it is compared. _Bird_ is a _genus_ when compared with the class _eagle_.
A _species_ is a class of things the name of which recalls more particulars than the name of another class or genus with which it is compared. _Bird_ is a _species_ when compared with the class _animal_.
In examining language, it will be found that the larger portion of words in common use are names of _genera_ and _species_--that is, they are words employed to recall ideas as they are arranged in genera and species. It is only those words that are _proper names_ which recall conceptions of the particular existences by which we are surrounded. Some of these surrounding existences are furnished with these particular names, and others can be designated and distinguished from each other only by a description. Thus we see some hills around our horizon, some of which have a peculiar name, and others can be designated only by describing the circumstances which distinguish them from all other hills.
A _definition_ of a word is an enumeration of the several qualities or circumstances which distinguish certain things from all others, and which are recalled to the mind when the word is used. Thus, if the word animal is to be defined, we do it by mentioning the circumstances of its _existence_ and _animal life_, as the ideas recalled by the word. Generally, a word is defined by mentioning the name of some _genus_ of which the thing intended is a _species_, and then adding those particular qualities which the species has, in addition to those included under the genus. Thus, if we are to define the word _man_, we mention the genus _animal_, and then the qualities which man has in addition to those possessed by other animals. Thus: "_Man_ is an _animal_, having the human form, and a spirit endowed with intellect, susceptibility, and will."
There are some words which recall only _one_ quality or circumstance, and which, therefore, can not be defined like the words which recall various qualities and circumstances, as joy, sorrow, color, and the like. Such words as these are defined by mentioning the times or circumstances when the mind is conscious of the existence of the idea to be recalled by the word. Thus _joy_ is "a state of mind which exists when any ardent desire is gratified." _Color_ is "a quality of objects which is perceived when light enters the eye."