 The main topics he deals with are: (1) the internal structure of the category; (2) the relationship between quality and what is qualified (quale); (3) the nature of the four species of quality Aristotle lists in the Categories (states and dispositions, natural capacities or incapacities to do or suffer something, affective qualities and affections, shapes and external forms); and (4) the distinctive characteristic (proprium) of the category of quality, i.e., the fact that two or more things can be described as similar only because of quality and in respect of it. We will focus on the first two only, since the questions they raise are more general.