Consider two different characters: Alanna and Brent. Both Alanna and Brent refuse to get the COVID-19 vaccine, but their motivations are different. Alanna believes that the vaccine is extremely unsafe and ineffective. Brent, on the other hand, believes that the vaccine is safe and effective. However, he is young and healthy, and he isn’t that worried about the effects that contracting COVID-19 would have on him. Some people have pointed out to Brent that even so, his getting vaccinated would help to protect others that he comes into contact with, some of whom are immunocompromised and so more vulnerable to the virus. However, Brent simply doesn’t care much about protecting others, and so he can’t be bothered to get vaccinated.Are these characters irrational? Many philosophers and social scientists, and perhaps many ordinary people too, would give different answers for Alanna and Brent. Alanna, they would say, is irrational. Let’s suppose, for the sake of argument, that Alanna has plenty of evidence that the vaccine is safe and effective: she’s seen the data that shows that the vaccinated are far less likely to die from COVID-19 than the unvaccinated, and that serious side effects of vaccination are extremely rare. If she continues to believe that the vaccine is extremely unsafe and ineffective, then, she has a belief that flies in the face of her evidence, and this seems like a paradigm case of irrationality. Brent, on the other hand, may be cruel and uncaring, but this—many would say—isn’t a matter of irrationality. Drawing on some arguments in my book, Fitting Things Together: Coherence and the Demands of Structural Rationality, I will suggest that this differential verdict—that Alanna is irrational but Brent is not—is not ultimately defensible. In the book, I distinguish two different kinds of rationality: substantive rationality, and structural rationality. Substantive rationality is about being reasonable, or responding correctly to your. . .