We investigate (co-) induction in classical logic under the propositions-as-types paradigm, considering propositional, secondorder and (co-) inductive types. Specifically, we introduce an extension of the Dual Calculus with a Mendler-style (co-) iterator and show that it is strongly normalizing. We prove this using a reducibility argument. 1 Introduction The Curry-Howard isomorphism. The interplay between logic and computer science has a long and rich history. In particular, the Curry-Howard isomorphism, the correspondence between types and propositions, and between typings and proofs, is a long established bridge through which results in one field can fruitfully migrate to the other. Classical calculi. One such example-motivating of the research presented herein-is the use of programming calculi based on Gentzen's sequent calculus LK [11]. At its core, LK is a calculus of the dual concepts of necessary assumptions and possible conclusions-concepts that map neatly, on the computer science side, to required inputs (or computations) and possible outputs (or continuations). As an example of the kind of analysis that can be done by focusing on this separation of concepts, Curien and Herbelin [6] and Wadler [22] devised LK-based calculi that showed, syntacticly, the duality of the two most common evaluations strategies: call-by-name and call-by-value. While originally such classical calculi included only propositional types-i.e. conjunction, disjunction, negation, implication and subtraction (the dual connective of implication)-they were later extended with second-order types [15, 20] and also with positive (co-) inductive types [15]. The latter fundamentally depended on the existence of a map operation of the underlying type. Here we do away with this restriction. Mendler induction. We take as primitive a more general induction scheme based on the work of Mendler [17]. He proposed an induction operator for System F that operated in a way akin to the callby-name fix-point combinator-thus avoiding direct use of said mapping operations-but ensured termination by clever use of polymorphic types. Further adding to its generality, it was later shown that with Mendler's iterator one could in fact induct on data-types of arbitrary variance-i.e. datatypes whose induction variable may also appear negatively [16, 21]. Due to its generality, Mendler induction has been applied in a number of different contexts, such as higher-order recursive types [1, 2] and automated theorem proving [13]. Classical logic and Mendler induction. The central question of this paper is whether Mendler induction can be lifted to non-functional settings without introducing ill-effects-specifically, non-Vol. 00, No.