This paper shows how classifications and definitions can be used to construct different patterns of logical reasoning called defeasible argumentation schemes, often identified with heuristics, or short-cut solutions to a problem. We show how it is possible to argue reasonably for and against arguments from classifications and definitions provided the arguments are seen as defeasible. We examine a variety of arguments of this sort, including argument from abductive classification, argument from causal classification, argument from analogy-based classification and arguments from classification based on generalizations. © Douglas Walton and Fabrizio Macagno.