So far I have explained how to divide categories and why categories are important. What I have not discussed is whether there is a lowest category and how to determine it if it exists. There are such lowest categories and they are determined by showing that there is no attribute capable of dividing the category in a fundamental manner.
I will begin by explaining what a lowest category is. In general a lowest category is a category of things that cannot be divided into different kinds of things. These ‘ lowest categories’ are called infima species. Specifically, it is a category such as humanity that cannot be divided into more specific secondary substances. Humanity cannot be divided by gender, race, origin, or any other means. Traditionally, humans are defined as rational animals. I think that it is more accurate to define humans as rational primates. (Primates include monkeys, apes, orangutans and similar animals.)
Remember that any division of a category must reflect a division in reality. Furthermore, any division must be a further specification of a previous division. So if human beings are divided further, it must be a further specification of rationality. Gender and race are a mixture of cultural and biological attributes. They are not further specifications of rationality. Origin offers no clear difference since it depends on historical attributes (such as born in Germany) or differences that are too specific (such as born to John and Mary). No other traits fail to have these sorts of problems.
In addition, none of these traits offer any fundamental explanatory advantage. Unless we can say that maleness changes everything so that we cannot understand particular human males without consider their maleness, then the trait is not of the right kind to divide that kind. Consider human beings. While maleness is necessary to understand the male hormonal system or the male side of reproduction, it is not necessary to understand male respiration (breathing). Male and female human beings breathe the same. Therefore, gender cannot legitimately divide human beings into more specific secondary substances.
Finally, remember that we are considering secondary substances in these cases. Secondary substances are not the same as attributes. They apply to the thing as a whole, not to some aspects of a thing. So we can only divide secondary substances further if that division explains every aspect of the thing when divided that way. If there is no attribute that divides the substance that way, then the secondary substance is the most specific secondary substance that it is possible to have. It is this that we refer to when we ask what something is.
If we were to consider attributes instead, then we follow the same rules. The ‘ thing’ under consideration is now an attribute. We have properly divided it when our division explains every aspect of that attribute so divided. If the division is not necessary to explain every aspect, then the attribute is not divided correctly.
This ends my discussion of language about matter and things. The next area of discussion is change.