  In 1922, however, Grabmann conclusively demonstrated that it was Thomas of Erfurt who wrote De modis significandi, not Duns  Scotus.[26]  So the claim that Heidegger wrote his Habilitationsschrift on Duns Scotus is only partly true. The first half, die Kategorienlehre, is correctly addressed to Scotus's theory of the categories as developed in his authentic commentaries on Porphyry's Isagoge, and Aristotle's Categories and De sophisticis elenchis. But the second half, die Bedeutungslehre, is based almost entirely on Thomas of  Erfurt.[27]