ABSTRACT: According to the so-called truth norm, we ought to believe that A if and only if A is true. There are many possible interpretations of this norm. What does ‘ought’ in this norm mean? Does ‘ought’ have a wide or a narrow scope, etc.? In this paper, I will investigate one version of this norm and I will discuss two arguments for it. The ‘ought’ in the paper will be interpreted as a kind of ‘rational’ ought that takes wide scope. I will call the first argument for the truth norm ‘the extrapolation argument’ and the second argument ‘the abductive argument.’ According to the extrapolation argument, we ‘derive’ the truth norm from a reflection on what it means to be a perfect believer. According to the abductive argument, the truth norm is supported by the fact that it can be used to deduce many other plausible doxastic norms. If this argument is successful, the truth norm can be conceived as the fundamental norm of (theoretical) rationality (or wisdom).