  Notions of unknown truths and unknowable truths are important in formal
epistemology, which are related to each other in e.g. Fitch's paradox of
knowability. Although there have been some logical research on the notion of
unknown truths and some philosophical discussion on the two notions, there
seems to be no logical research on unknowable truths. In this paper, we propose
a logic of unknowable truths, investigate the logical properties of unknown
truths and unknowable truths, which includes the similarities of the two
notions and the relationship between the two notions, and axiomatize this
logic.
