 Of those signs that are signs of something else (ii′*), some signify by convention (ad placitum, which Kilwardby proves by their not being the same for everybody); others signify naturally (like the sounds of non-rational animals). Of those words that signify by convention, some are simple expressions, and others are complex expressions (oratio or enuntiatio). Only significative complex expressions are true or false; simple expressions are neither true nor false (NSLPery I.2).