This paper develops an account of rigour in the use of thought experiments in ethics. I argue that there are two separate challenges to be faced. The first is internal validity: is the thought experiment designed in a way that allows its readers to make judgements that are confident and free of bias about the hypothesis or point of principle that it aims to test? The second is external validity: to what extent do ethical judgements that are correct of the world of the thought experiment generalize to a wide variety of other contexts, including ethical decision-making in the actual world? Ensuring external validity is the harder and more important problem of rigour, yet it is one that few philosophers have even noticed, let alone begun to solve.