Consciousness presents us with a number of different explanatory challenges. The most fundamental, sometimes called the ‘Hard Problem’, is focused on how subjective states can arise from objective physical systems. Another important question concerns the cognitive mechanisms that distinguish conscious from unconscious states. A third debate, and the one that will be the focus of the present enquiry, concerns how we can determine whether a given biological or artificial agent is conscious at all. Of the three questions, the latter has particular practical and ethical significance: our treatment of animals depends in part on whether we regard them as having a capacity for conscious experience.