Every word belongs to a group, and this group to some other larger group. A group is sometimes formed by derivation, at least so far as different meanings are marked merely by inflections, as _short_, _shorter_, _shorten_, _shortly_; but, for the most part, is a conflux of words from many different sources. _Repose_, _depose_, _suppose_, _impose_, _propose_, are not nearly connected in meaning; but are severally allied in sense much more closely with words philologically remote. Thus _repose_ is allied with _rest_, _sleep_, _tranquillity_; _disturbance_, _unrest_, _tumult_; whilst _depose_ is, in one sense, allied with _overthrow_, _dismiss_, _dethrone_; _restore_, _confirm_, _establish_; and, in another sense, with _declare_, _attest_, _swear_, _prove_, etc. Groups of words, in fact, depend on their meanings, just as the connection of scientific names follows the resemblance in character of the things denoted. 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.