It seems to be a platitude that what makes behaviour irrational is its failure to accord with some particular norm of rationality and it seems right to say that intentional action by and large conforms to these norms. These considerations might encourage one to attempt to explain an individual’s ability to act rationally, and account for some of her lapses, by attributing to her “knowledge” — either explicit or tacit — of what the norms require. The norms of rationality in some sense govern thought and action. But is the sense in which they do this captured by construing them as psychologically internalized rules, or as causal determinants of behaviour? The need to attribute some particular principle of rationality to an individual is defended by Davidson explicitly in his characterization of akrasia.1 I should like to explore his attempt to “individualize” the principle, or render it into a norm which is cognized by the individual whose actions are governed by it. This will require taking some space to explicate Davidson’s causal account of intentional action, which, for the sake of making the arguments clear, I shall just accept. I shall show that it is not necessary to individualize a principle of rationality in order to characterize an individual’s actions as internally irrational. In the second half of the paper I shall develop this argument by considering in detail what explanatory role an individual’s cognitive grasp of such norms might play. I shall argue that there is no construal of “cognitive grasp” such that attributing cognitivist grasp of a norm to an individual would explain her dispositions to act in accordance with what the norm prescribes, either directly, or via her second-order explicational abilities. I argue in the end that cognizing a norm of rationality could only be considered constitutive of an individual’s ability to obey it on a very artificial and stipulative sense of “obey”. I conclude that it is a mistake to construe the principles of rationality as norms or rules which may or may not be obeyed or followed.