 After Williams, the second most influential trope theorist is arguably Keith Campbell (1997[1981]; 1990). Campbell adopted the basics of Williams’ (standard) theory and then further developed and defended it. Later proponents of more or less standard versions of the trope view include Peter Simons, John Bacon, Anna-Sofia Maurin, Douglas Ehring, Jonathan Schaffer, Kris McDaniel, Markku Keinänen, Jani Hakkarainen, Marta Ujvári, Daniel Giberman and Robert K. Garcia (all of whose most central works on the topic are listed in the Bibliography). A very influential paper also arguing for a version of the theory (inspired more by Husserl than by Williams) is “Truth-Makers” (Mulligan et al. 1984; cf. also Denkel 1996). This paper defends the view that tropes are essentially dependent entities, the objects of perception, and the world’s basic truthmakers. Proponents of trope theories which posit tropes as one of several fundamental categories include C. B. Martin (1980), John Heil (2003), George Molnar (2003), and Jonathan Lowe (2006). Molnar and Heil both defend ontologies that include (but are not limited to) tropes understood as powers, and Lowe counts tropes as one of four fundamental categories. Even more unorthodox are the views of D. W. Mertz (1996, 2016), whose trope-like entities are categorized as a kind of  relation.[7]