The Attributives, classed under the title _accidens_, are regarded, like those classed under _differentia_, and _proprium_, as applicable to the class cut out by the _differentia_, but applicable to it rather fortuitously than by any fixed connection. The term _lame_ is an example of such Attributives. The term _lame_, however, applied to the name of the Species, does not the less take out of it a sub-species, as "lame man," "lame horse."
With respect to these classes of Attributives (_Differentia_, _Proprium_, _Accidens_) this is necessary to be observed, and remembered; that they differ from one another only by the accident of their application. Thus, when _rational_, applied to the Genus _animal_, constitutes the Species man, all other Attributives applied to that Species are either _accidens_, or _proprium_; but these Attributives themselves may be the _differentia_ in the case of other classes. Thus, _warm-blooded_, applied to _man_, stands under the class _proprium_; but { 168} when applied to the animals which stand distinguished from the cold-blooded, as constituting a class, it becomes the _differentia_, and _rational_, with respect to this comprehensive class, is only an _accidens_.[50]
[Editor's footnote 50: The author says, that no very distinct boundaries are marked by the three terms, Differentia, Proprium, and Accidens, nor do they effect a scientific division. As used, however, by the more accurate of the school logicians, they do mark out distinct boundaries, and do effect a scientific division.
Of the attributes common to a class, some have been taken into consideration in forming the class, and are included in the signification of its name. Such, in the case of man, are rationality, and the outward form which we call the human. These attributes are its Differentiæ; the fundamental differences which distinguish that class from the others most nearly allied to it. The school logicians were contented with one Differentia, whenever one was sufficient completely to circumscribe the class. But this was an error, because one attribute may be sufficient for distinction, and yet may not exhaust the signification of the class-name. All attributes, then, which are part of that signification, are set apart as Differentiæ. Other attributes, though not included among those which constitute the class, and which are directly signified by its name, are consequences of some of those which constitute the class, and always found along with them. These attributes of the class are its Propria. Thus, to be bounded by three straight lines is the Differentia of a triangle: to have the sum of its three angles equal to two right angles, being a consequence of its Differentia, is a Proprium of it. Rationality is a Differentia of the class Man: to be able to build cities is a Proprium, being a consequence of rationality, but not, as that is, included in the meaning of the word Man. All other attributes of the class, which are neither included in the meaning of the name, nor are consequences of any which are included, are Accidents, however universally and constantly they may be true of the class; as blackness, of crows.