1. Armstrong, D. M. (1980). Universals and scientific realism Vol. 1 and 2 (p. 2). Cambridge: Cambridge University Press.
2. Armstrong, D. M. (1989). Universals: An opinionated introduction. Boulder: Westview Press.
3. Ayers, Michael. (1997). Is physical object a sortal concept? A reply to Xu. Mind and Language, 12, 393–405.
4. Baghramain, M., & Carter, J.A. (2015). Relativism. In E. Zalta (Ed.), Stanford Encyclopedia of Philosophy.
5. Baker, M. (2003). Lexical Categories. Cambridge: Cambridge U. Press.