William Perry developed a theory of intellectual and ethical development based on an analysis of interviews with college students. The later positions in his scheme relate to thinking that is typical of what has been termed ‘post-formal operations’ or ‘a fifth stage of cognitive development’, indicating that it could be considered to extend the stage theory developed by Jean Piaget. Piaget’s ideas were highly relevant to two key features of many school science curricula—the logic of experimental testing, and the abstract nature of many science concepts. Perry’s work included students engaged in what could be considered more sophisticated thinking—such as engaging with multiple perspectives on a topic proposed by different theorists or reaching judgements when the available information is incomplete or inconsistent. This type of intellectual work is relevant to the school science curriculum in many national contexts where students are expected to appreciate both the provisional nature of scientific knowledge and something of the interactions of science with wider society. School students may be asked to discuss socio-scientific issues where scientific knowledge informs, but is not sufficient for, decision-making. Rather, scientific considerations must be coordinated with those deriving from broader value systems whilst taking into account the perspectives of different interest groups. Deanna Kuhn explored the development of children’s scientific thinking and critical thinking more generally. She identified a pattern that has strong parallels with Perry’s scheme.