“…But Aristotle never included this case into his own syllogistic, where subject terms are always affirmed. On the other hand, a number of contemporary works extended the famous square of opposition into a logical cube (Dekker 2015;Demey and Smessaert 2018;Dubois et al 2020), without always mentioning the central contribution to Keynes in this respect by revealing the 'other side' of categorical propositions (whenever the subject term is denied). Our point is not to implement these formal developments but, rather, to present a complete analysis of Keynes' octagon in three steps: first, a historical introduction to Keynes' logical achievements (Section 1); second, a look at the logical structure that was pioneered by Keynes (Section 2); and third, a formal semantics suggested to make sense of the logical relations between the eight propositions of Keynes' octagon.…”