Title: What Can Learned Intrinsic Rewards Capture?. Abstract: Reinforcement learning agents can include different components, such as policies, value functions, state representations, and environment models. Any or all of these can be the loci of knowledge, i.e., structures where knowledge, whether given or learned, can be deposited and reused. Regardless of its composition, the objective of an agent is behave so as to maximise the sum of suitable scalar functions of state: the rewards. As far as the learning algorithm is concerned, these rewards are typically given and immutable. In this paper we instead consider the proposition that the reward function itself may be a good locus of knowledge. This is consistent with a common use, in the literature, of hand-designed intrinsic rewards to improve the learning dynamics of an agent. We adopt a multi-lifetime setting of the Optimal Rewards Framework, and investigate how meta-learning can be used to find good reward functions in a data-driven way. To this end, we propose to meta-learn an intrinsic reward function that allows agents to maximise their extrinsic rewards accumulated until the end of their lifetimes. This long-term lifetime objective allows our learned intrinsic reward to generate systematic multi-episode exploratory behaviour. Through proof-of-concept experiments, we elucidate interesting forms of knowledge that may be captured by a suitably trained intrinsic reward such as the usefulness of exploring uncertain states and rewards.