"The historian of religions Ulf Drobin clarifies Trubetskoy's point: "all classification must stem from criteria. The followers of the language tree theory avoid definite criteria and replace them with a concept of language that is BOTH changeable (in time) and constant (Indo-European). In the final analysis they end up in paradoxes and mysticism. Ur-Indo-European must either lack prehistory, or it must have a non-Indo-European prehistory. The latter, however, cannot be explained with out some form of criteria" (Arvidsson 2006, p.297, emphasis and parentheses in the original)."