This paper advances a theory and method for evaluating political reasoning from a deliberative democratic perspective, as well as its transformative effects and outcomes. Deliberative reasoning ideally incorporates a process of wholly integrating relevant considerations (values and beliefs) into the formation of preferences for courses of action. In the absence of reasoning pathologies, groups engaged in deliberation tend to converge toward an intersubjectively shared pattern of reasoning. This manifests as a shared rationale: an embedded complex network of “if-then” relationships, or system of logic connecting considerations to preferences. A shared rationale is at least partly implicit and not necessarily verbalised in a comprehensive sense. Deliberation improved the integration of considerations into rationales. Under non-deliberative conditions, rationales become increasingly fractured and incorporate fewer considerations. A shared rationale is both constituted by and constituative of the deliberative situation, one that supports deliberative reflection, with individuals reasoning together to make sense of the world and what should be done. However, disagreement can (and should) still occur. Deliberative contestation can promote the development of ever more integrative rationales that incorporate a wider set of relevant considerations. Within a rationale, pluralism is only constrained by the logic that a rationale establishes. A rationale is socially constructed and cannot be easily modelled a priori, but it can be observed. A shared rationale implies a degree of consistency of (dis) agreement regarding considerations among pairs of individuals compared to (dis)agreement among their preferences, or Intersubjective Consistency (IC). Fourteen deliberative minipublic studies are presented as evidence, thirteen demonstrating an improvement in IC. The wider application of the method is also demonstrated by comparing reasoning among climate change sceptics to non-sceptics. The implications of the theory and findings for understanding and improving deliberative reasoning in wider political systems are discussed.