Path-dependency theory says that complex systems, such as health systems, are shaped by prior conditions and decisions, and are resistant to change. As a result, major policy changes, such as health system reform, are often only possible in policy windows—moments of transition or contextual crisis that re-balance social power dynamics and enable the consideration of new policy ideas. However, even in policy windows there can be resistance to change. In this paper, we consider the role of ideas in constraining change. We draw on political science theory on the dynamic relationship between foreground ideas (policy programmes and frames) and background ideas (deeply held collective cognitive and normative beliefs) to better understand how ideas exert influence independently of the contextual conditions that give rise to them or the actors that espouse them. To do so, we examine two apparent policy windows in the South African National Health Insurance policy process. The analysis reveals how ideas can become institutionalised in organisations and procedures (such as policy instruments or provider networks), and in intangible cultural norms—becoming hegemonic and uncontested ideas that shape the attitudes and perspectives of policy actors. In this way, ideas operate as independent variables, constraining change across policy windows. While health policy analysts increasingly recognise the influence of ideational variables in policy processes, they tend to conceptualise ideas as tools actors wield to drive change. This analysis reveals the importance of considering ideas (values, norms, and beliefs) as persistent features of the policy-making context that constrain actors.