In Categories, Aristotle claims that all things that exist are either complex or simple; and the simple things can be classified into ten categories.
The first and most basic category includes particular things such as this very chair in my room, or a water-pistol I once had; or indeed, a particular man such as Plato or particular woman such as Aspasia.
We can say what they are - their whatness - by giving their species ie a man or woman, or genus animal.
What distinguishes species from genus - other than by containment? And why two and not just one, or indeed more when one looks at the modern zoological classification.