A provocative chronicle of how US public health has strayed from its liberal roots. The Covid-19 response was a crucible of politics and public health—a volatile combination that produced predictably bad results. As scientific expertise became entangled with political motivations, the public-health establishment found itself mired in political encampment. It was, as Sandro Galea argues, a crisis of liberalism: a retreat from the principles of free speech, open debate, and the pursuit of knowledge through reasoned inquiry that should inform the work of public health. Across fifty essays, Within Reason chronicles how public health became enmeshed in the insidious social trends that accelerated under Covid-19. Galea challenges this intellectual drift towards intolerance and absolutism while showing how similar regressions from reason undermined social progress during earlier eras. Within Reason builds an incisive case for a return to critical, open inquiry as a guiding principle for the future public health we want—and a future we must work to protect.