[Note 18\3: _Hist. Sc. Ideas,_ b. viii. c. ii. sect. 3.] 14. But it may be asked, if we cannot define a {176} word, or a class of things which a word denotes, how can we distinguish what it does mean from what it does not mean? How can we say that it signifies one thing rather than another, except we declare what is its signification? The answer to this question involves the general principle of a natural method of classification, which has already been stated[19\3] and need not here be again dwelt on. It has been shown that names of _kinds_ of things (_genera_) associate them according to total resemblances, not partial characters. The principle which connects a group of objects in natural history is not a _definition_, but a _type_. Thus we take as the type of the Rose family, it may be, the common _wild rose_; all species which resemble this flower more than they resemble any other group of species are also _roses_, and form one _genus_. All genera which resemble Roses more than they resemble any other group of genera are of the same _family_. And thus the Rose family is collected about some one species, which is the type or central point of the group. [Note 19\3: _Hist. Sc. Ideas,_ b. viii. c. ii. sect. 3.] In such an arrangement, it may readily be conceived that though the nucleus of each group may cohere firmly together, the outskirts of contiguous groups may approach, and may even be intermingled, so that some species may doubtfully adhere to one group or another. Yet this uncertainty does not at all affect the truths which we find ourselves enabled to assert with regard to the general mass of each group. And thus we are taught that there may be very important differences between two groups of objects, although we are unable to tell where the one group ends and where the other begins; and that there may be propositions of indisputable truth, in which it is impossible to give unexceptionable definitions of the terms employed.