1. Berto F, Plebani M (2015) Ontology and metaontology. Bloomsbury, London
2. Bierling ER (1894) Juristische Prinzipienlehre, vol 1. Mohr Siebeck, Freiburg
3. Carnap R (1956) Meaning and necessity, 2nd edn. University of Chicago Press, Chicago
4. Dagger R, Lefkowitz D (2021) Political obligation. In: Zalta EN (ed) The Stanford encyclopedia of philosophy. https://plato.stanford.edu/entries/political-obligation
5. Fine K (1994) Essence and modality. Philos Perspect 8:1–16