  For the Modistae, then, the words ‘chimera’ and ‘horse’ differ, but only in terms of the complexity of their underlying modes. Thomas follows Siger of Courtrai in distinguishing between active and passive modi significandi/modi intelligendi to explain the difference between the act of signifying/understanding (materially construed as a property of the utterance/concept) and the object signified/understood (materially construed as a property of the thing signified/understood). The fact that nothing answers to the name ‘chimera’ simply does not matter. Modism was a theory about meaning (significatio) of a word as opposed to its reference (suppositio), and reference was regarded as something determined by logicians. Besides, if grammatical truths are universal and necessary — i.e., if there really is a science of grammar — then they cannot be altered by the fact that there are no chimeras. It is the assumption that some palpable phenomenon must underlie every grammatical truth, causing it to be the way it is, which guides Thomas's discussion in the remainder of the treatise, which soon moves on to more practical matters such as the different parts of speech (‘Etymologia’) and their syntax (‘Diasynthetica’). Included in the latter are the concepts of constructio (the syntactic joining of one word to another), congruitas (the proper formation of such constructions), and perfectio (the proper formation of complete expressions).