Stoic Sign-Inference and Their Lore of Fate (2024), p. 225
by Schumann, Andrew

Caption: The Stoic square of opposition for opposing two signs 'day', 'night' and two opposing signifieds 'light', 'dark'. There are the following differences from the Aristotelian square: (a) the bottom side of the square represents not a subcontrary relation, but a contradictory (['signified' $\oplus$ 'opposing signified'] $\equiv$ 'true'); (b) two diagonals are to fix not a contradictory relation, but a contrary (['sign' & 'opposing signified'] $\equiv$ 'false'; ['opposing sign' & 'signified'] $\equiv$ 'false'). The rest is the same: (c) the top side of the square is to show a contrary relation (['sign' & 'opposing sign'] $\equiv$ 'false'); (d) the two vertical sides of the square demonstrates a subalternation relation ('sign' $\Rightarrow$ 'signified'; 'opposing sign' $\Rightarrow$ 'opposing signified')