Philosophy Ph.D. Dissertations


Scientific Facts in the Space of Public Reason: Moderate Idealization, Public Justification, and Vaccine Policy Under Conditions of Widespread Misinformation and Conspiracism

Date of Award


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Philosophy, Applied

First Advisor

Kevin Vallier (Advisor)

Second Advisor

Christian Coons (Committee Member)

Third Advisor

Molly Gardner (Committee Member)

Fourth Advisor

Daniel Piccolo (Committee Member)


If liberal democratic theory requires that policy conform with citizens’ beliefs, then democracy seems to require bad policy when citizens hold false beliefs. To escape this problem, public reason liberals advocate epistemic idealization: Citizens’ false beliefs, bad inferences, and informational deficits are corrected in order to uncover the genuine reasons citizens hold. Politically legitimate policy must conform with citizens’ idealized reasons rather than their messy unreflective reasons. But advocates of idealization disagree over how much idealization is permissible. I focus on moderate idealizers like Gaus (2011) and Vallier (2014, 2018). They hold that the upward bound of idealization is set by the beliefs a real-world citizen could arrive at by sound deliberative route from their existing belief-value sets with a “reasonable” amount of effort.

Vallier and Gaus created their models before widespread social media use, echo chambers, high social and political polarization, and all the epistemic problems these create. For this reason, I argue, their models are understandably inadequate for addressing the vicious epistemic environments many citizens currently inhabit and the empirical beliefs they acquire from them. Contemporary moderate idealizers should adopt the exclusion principle whereby we permissibly exclude from policy considerations deeply held empirical beliefs when they contradict a consensus of relevant experts in a mature science—even if they survive moderate idealization. Incorporating this principle generates better policy outcomes and better supports pre-theoretical intuitions about political legitimacy.

Chapter 1, argues that, under these conditions, Vallier’s moderate idealization leads to normatively and epistemically bad policy, and that the exclusion principle solves this problem from within the commitments of political liberalism. Chapter 2 argues that Gausian moderate idealization also leads to normatively and epistemically bad policy when epistemically vicious environments are widespread. Finally, Chapter 3 applies these arguments to vaccine policy and demonstrates that (a) neither Gaus nor Vallier’s models generate acceptable publicly justified immunization policy; (b) moderate idealizers must adopt the exclusion principle in order to do so and, more broadly, to satisfy pre-theoretical intuitions about political legitimacy; and (c) the exclusion principle can be justified from within public reason liberalism.