Can post-truth thinking be rational? In order to answer that question I develop, in the first part of this article, a non-pejorative understanding of post-truth thinking, namely as the systematic underestimation of the epistemic value of the expert discourse as compared to ones individual deliberation in relation to politicized factual issues in an environment without secure epistemic rules. Everyone significantly underestimates how more reliable academic discourse, say, is compared to individual epistemic means.
In post-truth thinking this underestimation concerns questions the answers to which allow for predictions about political affiliation. In answering such questions about the truth of the theory of evolution, sayalmost everyone has to draw on the testimony of others one regards as being trustworthy. Oftentimes one finds these trustworthy people in his or her social media filter bubbles. Post-truth thinking happens when one has to inform oneself in social or alternative media for which we currently lack safe epistemic rules of thumb or heuristics. “Post-truth thinking” seems to imply indifference about truth or rationality. Against this assumption I argue, in the second part, that post-truth thinking can be regarded as being rational, at least in the sense of “bounded rationality”. After all, everyone has to rely on the testimony of others in almost all fields of knowledge. In non-ideal circumstances, which are characteristic for post-truth thinking, it is rational, in navigating social and alternative media, to follow epistemic rules well-established in other domains. These rules often speak for believing what emerges in ones filter bubble.