1. Armstrong, D. M. (1988). Can a naturalist believe in universals? In E. Ullmann-Margalit (Ed.), Science in reflection (pp. 103–115). Dordrecht: Kluwer Academic Publishers.
2. Butchvarov, P. (1979). Being qua being: A theory of identity, existence, and predication. Bloomington: Indiana University Press.
3. Ehring, D. (2002). Spatial relations between universals. Australasian Journal of Philosophy, 80, 17–23.
4. Grossmann, R. (1992). The existence of the world: An introduction to ontology. London: Routledge.
5. Hoffman, J., & Rosenkrantz, G. (1997). Substance: Its nature and existence. London: Routledge.