Acknowledgements
- The illustration on the left is a reproduction of M.C. Escher's woodcut print "Sky and Water I" (1938). We thank the The M.C. Escher Company, the Netherlands, for granting us permission to use it.
- We thank the Agence Nationale de la Recherche and ANR Program "Cognitive Origins of Vagueness" for funding, as well as the Formal Epistemology Project/University of Leuven, funded by the Flemish government (Odysseus grant) and the Research Council/University of Leuven (CREA grant).