_Posted on June 28, 2025_ ##### Introduction Building upon my commentary on [[Categories, Chapter 3a - Dictum de omni et nullo|Categories, chapter 3a]], recall that abstraction involves _extracting_ predicates from similarities among individuals. In using such an extracted predicate, the focus shifts towards particular commonalities between individuals. Natually, this results in overall information loss (however, as noted in [[Categories, Chapter 3a - Dictum de omni et nullo|Categories, chapter 3a]], this usage has utility in discourse). In chapter 3b, the terms "genus" and "species" are first used. We may say that a "genus" is the class which some abstracted predicate forms, while a species is similarly such a class, though directly under some genus. To understand Aristotles use of these terms, it is helpful to consider Porphyry's explaination of how genus and species function in a complete heirarchy: > "There is a highest genus beyond which there can be no other superior genus; there is a lowest species after which there can be no subordinate species; and between the highest genus and the lowest species there are some classes which are genera and species at the same time, since they are comprehended in relation to the highest genus and to the lowest species. The intermediate classes will be species of prior classes but genera of posterior classes." > - Porphyry, *Isagoge*, trans. Edward W. Warren (Toronto: The Pontifical Institute of Mediaeval Studies), 36. --- ##### The Text >The differences of different genera, and of things not arranged under each other, are diverse also in species >(1b16-1b17) Under the above framework, the differences of a genus would be second-order information differentiating between the kinds of individuals within that genus. Aristotle is conveying here that different genera have different "differences", and corresponding "species". That is, the kind of information lost in predicating an individual of its genus is different for different genera, and that as a result, different genera have different species. Predicating of an individual its species restores some of the information lost in our initial abstraction, while moving "down" a hierarchy of predicates. >as of "animal" and "science". For the differences of "animal" are "quadruped," "biped," "winged," "aquatic," but none of these, forms the difference of "science," since "science," does not differ from "science," in being "biped." >(1b18-1b20) The ideas of "animal" and "science" are clearly abstracted concepts. When asserting that some thing is animal, we lose information such as whether it is "'quadruped', 'biped', 'winged', [or] 'aquatic'", but this is not the same kind of information we lose in asserting that some thing is "science". > But as to subaltern genera, there is nothing to prevent the differences being the same, as the superior are predicated of the genera under them; so that as many differences as there are of the predicate, so many will there also be of the subject. > (1b21-1b24) Subaltern genera are those genera which are under some other genus. In other words, a subaltern genus is a species with respect to a higher genus, but a genus with respect to its own species. The quoted sentence simply asserts that the differences of a genus are inherited by its species. Any difference which a predicate has are also differences of the subject. Therefore, since a genus can be predicated of its species, the differences of the genus are also differences of the species.