 Kilwardby argues that only human beings proffer meaningful utterances because, as Priscian points out, the articulate vocal sound (uox articulata) uttered by humans is tied together (copulata) with a sensus mentis or intellectus in the mind of the speaker; thus only beings that have minds can produce significative  utterances.[26]  Moreover, the production of the vox litterata requires the control of the modulation of sounds—what Kilwardby calls the figure in the air (through which the utterances are transmitted)—on a regular basis, that is, always the same figure for the same word. This is done by the sensory soul under the control of the rational soul (NLPA, 124–125). This explains not only why meaningful articulate vocal sounds are signs but also their primacy as efficiently caused by the soul (see Mora-Márquez 2014).