 Socrates and Plato are real; their agreement is real, too, but it isn’t to be explained by appealing to any thing—their agreement just is their each being human. From a metaphysical point of view they have the same standing as human beings; this does not involve any metaphysically common shared ingredient, or indeed appeal to any ingredient at all. That is the sense in which there is a “common reason” for the imposition of a common name. 