Practical knowledge and error in action

G.E.M. Anscombe's account of practical knowledge raises a puzzle for cases of practical error, i.e. cases where, due to a mistake of mine, I am not in fact doing what I mean to be doing. It can seem that in such cases, we must both affirm and deny the presence of practical knowledge. It must be present, because practical error presupposes that there is an intentional action in which the mistake occurs, which in turn presupposes practical knowledge as a formal-causal condition. At the same time, practical knowledge can't be present, because I am not doing what I think I am doing, and therefore can't have knowledge of what I am doing. I discuss and reject various attempts to deal with the problem. The solution I propose and defend against objections relies on applying the Aristotelian-Thomistic conceptual framework of form and matter to intentional action.