Each concrete general term has two _meanings_, (1) the actual concrete thing, person or object to which the term is applied; and (2) the qualities, attributes or properties of those objects, persons or things in consequence of which the term is applied. For instance, in the case of the concrete term _book_, the first meaning consists of the general idea of the thing which we think of as _a book_, and the second meaning consists of the various qualities which go to make that thing a book, as the printed pages, the binding, the form, the cover, etc. Not only is that particular thing _a book_, but every other thing having the same or similar properties also must be _a book_. And so, whenever I call a thing _a book_ it must possess the said qualities. And, whenever I combine the ideas of these qualities in thought, I must think of _a book_. As Jevons says: "In reality, every ordinary general term has a double meaning: it means the things to which it is applied, ... it also means, in a totally different way, the qualities and peculiarities _implied_ as being in the things. Logicians say that the number of things to which a term applies is the _extension_ of the term; while the number of qualities or peculiarities implied is the _intension_." The extension and intension of terms has been referred to in the previous chapter. The general classification of the degrees of _extension_ of a general term is expressed by the two terms, _Genus_ and _Species_, respectively. The classification of the character of the _intension_ of a term is expressed by the term, _Difference_, _Property_ and _Accident_, respectively. _Genus_ is a term indicating: "a class of objects containing several species; a class more extensive than a species; a universal which is predicable of several things of different species." _Species_ is a term denoting: "a smaller class of objects than a genus, and of two or more of which a genus is composed; a predicable that expresses the whole essence of its subject in so far as any common term can express it."