Words, accordingly, stand related to one another, for the most part, though very irregularly, as genus, species, and co-ordinate species.  Taking repose as a genus, we have as species of it, though not exactly co-ordinate with one another, tranquillity with a mental differentia (repose of mind), rest, whether of mind or body, sleep, with the differentia of unconsciousness (privative).  Synonyms are species, or varieties, wherever any difference can be detected in them; and to discriminate them we must first find the generic meaning; for which there may, or may not, be a single word.  Thus, equality, sameness, likeness, similarity, resemblance, identity, are synonyms; but, if we attend to the ways in which they are actually used, perhaps none of them can claim to be a genus in relation to the rest.  If so, we must resort to a compound term for the genus, such as ’absence of some sort of difference.’  Then equality is absence of difference in quantity; sameness is often absence of difference in quality, though the usage is not strict:  likeness, similarity, and resemblance, in their actual use, perhaps, cannot be discriminated; unless likeness be the more concrete, similarity the more abstract; but they may all be used compatibly with the recognition of more or less difference in the things compared, and even imply this. Identity is the absence of difference of origin, a continuity of existence, with so much sameness from moment to moment as is compatible with changes in the course of nature; so that egg, caterpillar, chrysalis, butterfly may be identical for the run of an individual life, in spite of differences quantitative and qualitative, as truly as a shilling that all the time lies in a drawer.

Co-ordinate Species, when positive, have the least contrariety; but there are also opposites, namely, negatives, contradictories and fuller contraries.  These may be regarded as either co-ordinate genera or the species of co-ordinate genera.  Thus, repose being a genus, not-repose is by dichotomy a co-ordinate genus and is a negative and contradictory; then activity (implying an end in view), motion (limited to matter), disturbance (implying changes from a state of calm), tumult, etc., are co-ordinate species of not-repose, and are therefore co-ordinate opposites, or contraries, of the species of repose.

As for correlative words, like master and slave, husband and wife, etc., it may seem far-fetched to compare them with the sexes of the same species of plants or animals; but there is this resemblance between the two cases, that sexual names are correlative, as ‘lioness,’ and that one sex of a species, like a correlative name, cannot be defined without implying the other; for if a distinctive attribute of one sex be mentioned (as the lion’s mane), it is implied that the other wants it, and apart from this implication the species is not defined:  just as the definition of ‘master’ implies a ‘slave’ to obey.