This is the reason why, though we cannot believe it impossible to God to make a creature with other organs and more ways to covet into the understanding the notice of corporeal things than those five, as they are usually counted, which he has given to man; yet I think it is not possible for anyone to imagine any other qualities in bodies, however constituted, by which they can be taken notice of, besides sounds, tastes, smells, visible, and tangible qualities. And had mankind been made but with four senses, the qualities then, which are the objects of the fifth sense, had been as far from our notice, imagination, and conception, as now any belonging to a sixth, seventh, or eighth sense can possibly be.
~ Locke, an essay concerning human understanding, chapter II, par.3
It is interesting to me to explore the extent and limitations the human situation sets to the study of modality. Is it logically possible for man to have a ninth sense? What exactly would we be saying if we answered in the affirmative? Would we be saying that there is some logically possible world which satisfies the proposition “man has nine senses”? But, since that picture of the world is psychologically and nomologically beyond the perview of human imagination, there is no way for man to model a conceptual state of affairs which satisfies having a ninth sense.