Authorship attribution often suffers from charges of vicious circularity, of conflating authorship and style. We argue for a more virtuous circularity for attribution arguments made through the quantitative analyses of stylometry. Our study investigated temporal lexeme usage, which has been suggested as a mark of authorship style, in the corpus traditionally attributed to Chrétien de Troyes. Our analyses revealed different usage patterns for these lexemes after line 6150 of the Lancelot, possibly indicating the signature of another author in the text (traditionally presumed to be that of Godefroy de Lagny). By embracing probabilistic arguments instead of certainties, digital philology can employ stylometry to consider questions of authorship attribution for medieval texts.