[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. The author's remark, that these three classes of Attributives differ from one another only in the accident of their application, is most just. There are not some attributes which are always Differentiæ, and others which are always Propria, or always Accidents. The same attribute which is a Differentia of one genus or species, may be, and often is, a Proprium or an Accidens of others, and so on.--_Ed._]