Although neoclassical economics faces frequent criticism, it remains the domi- nant paradigm, largely due to its immunisation strategies that rely on unfalsifia- ble concepts of utility and rationality. In this paper, I use Karl Popper's philoso- phy to assess whether these strategies are justified. Firstly, I reconstruct Popper's ideas on immunisation strategies, situational analysis, the rationality principle, and the metaphysical research programme. Next, I examine how neoclassical eco- nomics' immunisation strategies counter critiques from behavioural economics. I conclude that neoclassical economics' method does not produce empirical con- jectures. I assess and evaluate this finding in relation to the "rationality principle", as it parallels neoclassical economics' optimisation axiom. Furthermore, I explore Popper's relevance in the context of neoclassical economics' recent incorporation ofbehavioural economics' insights to show that my analysis is not purely historical.