The terra incognita spaces on maps say that knowledge also is an island surrounded by oceans of the unknown. They signify that the cartographers knew they did not know, and awareness of ignorance is not just ignorance; it's awareness of knowledge's limits. [...] To acknowledge the unknown is part of knowledge, and the unknown is visible as terra incognita but invisible as selection.

Rebecca Solnit, A Field Guide To Getting Lost (Canongate, 2006) p. 163