Last time, we ended with Locke’s thoughts about substances. Substances are the things that have the qualities we perceive. But since they are distinct from their qualities, and the qualities are the only things we are aware of, we do not know anything about them. The closest we come is what he called complex ideas, that is, ideas that group a number of simple qualities as belonging to one thing.
Today, we are talking about kinds of things, that is, abstract categories. This table in my house is just as much a table as every table in Pearsons Hall. They all belong to one kind or category: tables. Locke uses the word “sorts” for “kinds” or “categories.”
Locke is a nominalist. The world contains particular things and we make the categories or, to use his term, we do the sorting.
That said, there are real, underlying reasons why things fall into the sorts that they do. But we do not know what those reasons are.
He expresses this idea with his distinction between real and nominal essences. Nominal essences reflect our thoughts and we use them to sort things into categories. Real essences are the underlying reason why things are the way they are but we do not know them and so they are not relevant to the categories we use.