  But this is where things get interesting, for the Modistae also understood the theory of the modi significandi to involve claims about the nature of thought and reality. The problem with Priscian, as twelfth-century grammarians had pointed out, is that he said nothing about what causes the different parts of speech. Hence the need for a theory of the modi significandi. But the Modistae saw that this could only be part of the answer, for grammar is a linguistic phenomenon, and linguistic phenomena must have a cause within the natural order of things. Therefore, to complete the explanation, they argued that the formal structure of the modi significandi owes its existence to modi intelligendi, or modes of understanding, which in turn are caused by modi essendi, or the modes of being a thing can exhibit outside the mind.