Why There Is a Vestibular Sense, or How Metacognition Individuates the Senses.

Should the vestibular system be counted as a sense? This basic conceptual question remains surprisingly controversial. While it is possible to distinguish specific vestibular organs, it is not clear that this suffices to identify a genuine vestibular sense because of the supposed absence of a distinctive vestibular personal-level manifestation. The vestibular organs instead contribute to more general multisensory representations, whose name still suggest that they have a distinct 'sensory' contribution. The vestibular case shows a good example of the challenge of individuating the senses when multisensory interactions are the norm, neurally, representationally and phenomenally. Here, we propose that an additional metacognitive criterion can be used to single out a distinct sense, besides the existence of specific organs and despite the fact that the information coming from these organs is integrated with other sensory information. We argue that it is possible for human perceivers to monitor information coming from distinct organs, despite their integration, as exhibited and measured through metacognitive performance. Based on the vestibular case, we suggest that metacognitive awareness of the information coming from sensory organs constitutes a new criterion to individuate a sense through both physiological and personal criteria. This new way of individuating the senses accommodates both the specialised nature of sensory receptors as well as the intricate multisensory aspect of neural processes and experience, while maintaining the idea that each sense contributes something special to how we monitor the world and ourselves, at the subjective level.