This paper is ultimately about the nature of argumentation in general and about the nature of practical argumentation in particular. (Practical argumentation is the form of argumentation which aims at answering the question: ‘What is to be done?’) The approach adopted here is an indirect one. I analyze one traditional form of argumentive fallacyargumentum ad hominem and try to show that in some argumentative situations it is an intuitively legitimate move. These intuitions can be explained if we accept that practical argumentation is also governed by the general rules of practical rationality.