Some variety of realism seems inescapably to be true. Regarding whether each Person of the Trinity is a “thing” (res), St. Anselm points out several analogies against the nominalist Roscellin of Compiegne, who asserted (apparently without much thought) that the three Persons of the Trinity were three separate “things”:
First, several men (individuals) are one man (species). Second, one individual horse of a particular color is separable from the color itself in that another individual horse could also be that color. Third, “fatherhood” is a common property of a father and his son when the latter has his own son–that is, the relationships described by grandfather–>father–>grandson. Fourth, three individual men can at different times and places all be called “king,” and “king” means the same thing in all three individual cases. Fifth, considering a spring that creates a river which accumulates into a lake, all three can bear the same name, “Nile,” without there being three distinct “Niles” in an absolutely self-contained sense.:”(“The Incarnation of the Word,” in Complete Philosophical and Theological Treatises of Anselm of Canterbury, trans. Jasper Hopkins and Herbert Richardson [Minneapolis, MN: The Arthur J. Banning Press, 2000], pp. 267-294.)”:
In similar fashion, “God” can be that which is common to “Father,” “Son,” and “Holy Spirit,” each of which are distinct persons yet the same substance. Whatever nominalism may have going for it in other ways, if it is spun as the radical assertion that the only things which exist are totally independent and self-contained individual things, it is simply false, as human experience of the world by itself shows.