 Similarly, he started to work with psychologists on a model of psychological '(de)activation' for phonological processes and, with his background in IE studies, he compared historical evidence with his phonological theory, making conclusions about rules, processes and the boundaries of phonological theory towards morpho(no)logy.In his contributions in morphology, Dressler established, together with Wolfgang Wurzel and Oswald Panagl, a subtheory of 'Natural Morphology' based on the establishment of more or less "natural" operations on universal, typological, or language-specific levels, respectively.In a monograph on morphonology, he proved morphonology to be a subtype of morphology, contrary to contemporary claims in generative phonology of its being treated as a phonological phenomenon.