The main subject of the study is the criticism of Aristotelian categorical system proposed by Plotinus. Treatises VI 1-3 focus on the problem of the possibility of thought within particular system of categories. Plotinus identifies categories with the kinds of being, being based on the tradition of such identification originated in the Plato’s “Sophist”. In some texts Aristotle himself speaks of categories as the kinds of being. The article is divided into two main parts. The first part is about Aristotle’s (...) position. I discuss the following questions: 1) what does the common name “being” mean; 2) why may the categories be the kinds of being and why should the substance be the first among the categories; 3) how are the substance and other categories related? The second part is about Plotinus’ criticism. Plotinus asserts 1) that not the substance, but being is the subject matter of the first philosophy, 2) that ten categories are not equally applicable to the intelligible world and to the world of sense, and 3) that the substance has two different meanings with respect to these worlds, and therefore the substance can’t be one kind, which includes the corporeal and incorporeal substances. (shrink)