Abstract

Why must we respect others’ rights to dispute scientific knowledge such as that the Earth is round, or that humans evolved, or that anthropogenic greenhouse gases are warming the Earth? In this paper, I argue that in On Liberty Mill defends the freedom to dispute scientific knowledge by appeal to a novel social epistemic rationale for free speech that has been unduly neglected by Mill scholars. Mill distinguishes two kinds of epistemic warrant for scientific knowledge: 1) the positive, direct evidentiary warrant that scientific experts construct for their knowledge by applying the methods Mill had set out in his A System of Logic, Ratiocinative and Inductive, and 2) a social testimonial warrant that the non-expert “public” has for what Mill refers to as their “rational[ly] assur[ed]” beliefs on scientific subjects. Mill does not argue that scientific claims can never be proven true with complete practical certainty to scientific experts, nor does he argue that scientists must engage in free debate with critics such as flat-earthers in order to fully understand the grounds of their scientific knowledge. Instead, Mill argues that in the absence of the freedom to dispute scientific knowledge, non-experts cannot establish that scientific experts are credible sources of testimonial knowledge. To establish the credibility of scientific expert speakers, non-expert audiences must have a rational assurance, Mill argues, that experts have satisfactory answers to objections that might undermine the positive, direct evidentiary proof of scientific knowledge. But since non-experts cannot distinguish objections that undermine such expert proof from objections that do not, censorship of any objection — even the irrelevant objections of literal or figurative flat-earthers — will prevent non-experts from determining whether scientific expert speakers are credible. Hence, while censoring irrelevant objections would not undermine the positive, direct evidentiary warrant that scientific experts have for their knowledge, doing so would destroy the non-expert, social testimonial warrant for that knowledge. The asymmetry between how expert scientific speakers and non-expert audiences warrant their scientific knowledge is what both generates and necessitates Mill’s social epistemic rationale for the “absolute” freedom to dispute it.