In this chapter, the central philosophical pursuit of theoretical linguistics is recentred around the issue of what makes a language a possibly human or natural one. Thus, instead of discussing the old immovable debates concerning the ontology of language in terms of nominalism, mentalism, and platonism, we move the discussion to modal metaphysics. However, this isn’t an exercise in abstract a priori reasoning. The space of possibilities are deeply constrained by current linguistic theory. Besides proffering a novel conception of language and linguistic possibility, the concepts of ‘naturalness’ in philosophy, ‘learnability’ in generative linguistics, and ‘communication’ in the philosophy of language are discussed. The idea is that linguistic possibility or possible languages, like possible worlds, can be defined in terms of accessibility relations from actual languages based on their learnability, use as communicative devices, and a number of other possible constraints. Lastly, the chapter critiques some influential experimental work in neurolinguistics on the notion of impossible languages, which rely more heavily on generative biolinguistics. These experiments, although interesting, don’t show what they intend to. Linguistic impossibility is best derived from its possibility through the method presented in the chapter.

It is commonly held in many philosophy of quantum gravity circles that endorsing Lewis ontology of modal realism is incompatible with endorsing the fundamental physical ontology of any quantum gravity theory. The unsolvable tension would be between the Lewis metaphysical commitment to the fundamentality of space and time and the physical lesson of quantum gravity about the disappearance of space and time from the fundamental structure of the world. In this essay I argue against the idea that the tension is unsolvable. This analysis does not apply to quantum gravity in genera, but only to quantum string theory.