11. ‘The Five Words.‘—These things,—the Nature and Relations of Classes,—were, in fact, the subjects of minute and technical treatment by the logicians of the school of Aristotle. Porphyry wrote an Introduction to the Categories of that philosopher, which is entitled On the Five Words. The ‘Five Words’ are Genus, Species, Difference, Property, Accident. Genus and Species are superior and inferior classes, and are stated[2] to be capable of repeated subordination. The ‘most [106] general Genus’ is the widest class; the ‘most special Species’ the narrowest. Between these are intermediate classes, which are Genera with regard to those below, and Species with regard to those above them. Thus Being is the most general Genus; under this is Body; under Body is Living Body; under this again Animal; under Animal is Rational Animal, or Man; under Man are Socrates and Plato, and other individual men.
[2] Porphyr. Isagog. c. 23.
The Difference is that which is added to the genus to make the species; thus Rational is the Difference by which the genus Animal is made the species Man; the Difference in this Technical sense is the ‘Specific,’ or species-making Difference[3]. It forms the Definition for the purposes of logic, and corresponds to the ‘Character’ (specific or generic) of the Natural Historians. Indeed several of them, as, for instance, Linnæus, in his Philosophia Botanica, always call these Characters the Difference, by a traditional application of the Peripatetic terms of art.
[3] εἰδοποιός.
Of the other two words, the Property is that which though not employed in defining the class, belongs to every part of it[4]: it is, ‘What happens to all the class, to it alone, and at all times; as to be capable of laughing is a Property of man.’
[4] Isagog. c. 4
The Accident is that which may be present and absent without the destruction of the subject, as to sleep is an Accident (a thing which happens) to man.
I need not dwell further on this system of technicalities. The most remarkable points in it are those which I have already noticed; the doctrine of the successive Subordination of genera, and the fixing attention upon the Specific Difference. These doctrines, though invented in order to make reasoning more systematic, and at a period anterior to the existence of any Classificatory Science, have, by a curious contrast with the intentions of their founders, been of scarcely [107] any use in sciences of Reasoning, but have been amply applied and developed in the Natural History which arose in later times.
We must now treat of the principles on which this science (Natural History) proceeds, and explain what peculiar and technical processes it employs in addition to those of common thought and common language.