This paper examines the properties of the validity and scope of arguments as standards for evaluating the epistemic qualities of particular deliberative exchanges within a context of value pluralism where parties can hold differing views of the common good based on incommensurable basic values. In this context, the task of political decisions is to maximise the interests of all, only judging between internally coherent versions of the common good on the basis of their mutual impact. The paper argues open, democratic, (...) deliberation provides the best chance of discovering such a path, through developing mutual understanding across incommensurable positions. Identifying the validity and scope of a deliberative exchange enhances this potential. Any deliberative exchange may be characterised in terms of its validity – the reasons that support its conclusions – and scope – the unquestioned assumptions that mark the boundaries of that reasoning. The paper explains the epistemic role of these two features in developing understanding across plural positions. Clarity about the validity of an exchange reveals positions that can be shared. Acknowledgement of its scope avoids over-extending its conclusions and reveals how positions differ, providing the starting point for further deliberation about their co-ordination. The paper ends by drawing the conclusion that deliberation characterised in this way should be democratic and open. (shrink)