The distinction between formal and informal logic is clarified as a prelude to considering their ideal relation. Aristotle's syllogistic describes forms of valid inference, and is in that sense a formal logic. Yet the square of opposition and rules of middle term distribution of positive or negative propositions in an argument's premises and conclusion are standardly received as devices of so-called informal logic and critical reasoning. I propose a more exact criterion for distinguishing between formal and informal logic, and then (...) defend a model for fruitful interaction between informal and formal methods of investigating and critically assessing the logic of arguments. (shrink)