 With ideas, the crucial question was whether or not a certain object is indeed represented by them; the corresponding question for propositions is whether or not they are true. Just as I have called ideas compatible or incompatible with each other, depending on whether or not they have certain objects in common, so I call propositions compatible or incompatible, depending on whether or not there are certain ideas which make all of them true. (TS II: §154, 101; Bolzano 1973: 198–199)