This paper introduces Prior’s unpublished paper Aristotle on Logical Squares, which is deposited in the Bodleian Library and which discusses Greniewski’s definition of the \ operator, which Greniewski introduced in his paper Próba ‘odmłodzenia’ kwadratu logicznego. It is a unique attempt to formalize the square of opposition. Bendiek’s review, which is an important intermediary between Greniewski’s and Prior’s paper, is also mentioned here. Greniewski’s main motivation was to rejuvenate the traditional square of opposition in order to make a square of (...) opposition more precise. He did not investigate the history of the square of opposition. Prior’s approach differs a great deal from Greniewski’s one because he bases his analyses of Greniewski’s operator on Aristotle’s definitions of squares of oppositions. There are some queries, which arise due to the different aims of Greniewski and Prior and their different approach might have caused Prior’s effort to the approximation of Greniewski’s operator to Aristotle’s definitions of the squares of opposition was unsuccessful. (shrink)