Theodosius’ Κανόνες εἰσαγωγικοὶ περὶ κλίσεως ὀνομάτων καὶ ῥημάτων ‘Elementary rules on the inflection of nouns and verbs’ (4th ct. AD) is a school work devoted to the teaching of nominal and verbal inflections. Besides full paradigms, this work provides a complex set of synchronic rules for declensions and conjugations, thus producing an impressive amount of otherwise unattested, bizarre and even impossible forms. Partly owing to this inclusiveness, the Κανόνες - which for many centuries have played a remarkable role in the teaching of Greek - do not enjoy a very high reputation today. However, at closer inspection they may prove of some interest, as they offer a clue for reconsidering aspects of Ancient Greek from an unusual perspective - “through ancient Greek eyes”, so to speak. A case study is offered in this paper, focusing on the treatment of the perfect, and on the intersection between morphological and “functional” criteria in the constitution of the system reported in the Κανόνες, which opposes an active, a middle and a passive perfect (e.g. tétupha, tétupa, tétummai, from túptō ‘strike’). This seemingly bizarre and obscure tripartite system is here interpreted in the light of a double opposition: between diathetically indifferent forms (i.e. hosting both active and passive functions, hence “middle”, according to ancient grammarians) and diathetically non-indifferent ones; then, among the latter, between “active” and “passive” forms. The Κανόνες thus highlight and project onto a purely synchronic and highly artificial system the effects of crucial developments of the perfect in Greek.