Invalidity of “Subject-Obversion” Rule

Document Type : Research Paper


Assistant Professor in Islamic Philosophy at University of Tehran


In contemporary texts of Islamic logic, the “subject-obversion” has been introduced as a rule on basis of which one can replace the subject term of a categorical proposition with its complement and change both quality and quantity of the proposition. Thus from an A-proposition such as “all A is B”, one can infer “Some not-A is not B”. Well, let’s take that A is included by B but not vice versa, and B is most extensive universal such as “existent” or “thing”. In this case “all A is B” produces the contradictory of our favorite conclusion, i.e. “all not-A is B”. Therefore, the so-called subject-obversion rule, at least in A-proposition, is not logically valid.