Philosophers disagree about the role of reflection for rationality, understood as the capacity to (properly) respond to genuine, normative reasons. Here, ‘reflection’ means the capacity for self-conscious normative meta-cognition. This article develops and rejects a novel argument – the argument from undercutting defeaters – in favor of the ‘one-level view’ that holds that having the concept of a belief (and of a reason) is necessary for responding to reasons. It will be argued that the ‘two-level view’, which allows for rational (...) subjects that can only non-reflectively respond to reasons, is supported by considerations dealing with the role of responding to reasons for rational action. Rationality is not as unified as the one-level view wants to have it. We start with the non-reflective way of rationally forming beliefs and then grow into the reflective way. (shrink)