We provide a characterization of knowledge resistance in terms of resistance to available evidence and a philosophical guide to the concepts central to empirically investigating it: knowledge, evidence, and rationality. Knowledge requires true, justified belief, so we emphasize the importance of focusing on factual judgements the truth of which can be investigated by empirical methods. We understand evidence in terms of probabilification, and discuss its content and its testimonial nature in the central cases. We propose that knowledge resistance always involves epistemic irrationality. An important psychological mechanism resulting in such irrationality is motivated reasoning, and politically motivated reasoning has been proposed as the main explanation of fact polarization. We discuss challenges to the detection of motivated reasoning, stressing the rationalizing role of prior belief. When priors line up with motivations, these two factors are difficult to disentangle. But even where polarization results from differences in prior belief, there might be irrationality, for instance in the form of unjustified beliefs about which sources of evidence are trustworthy. Therefore, we propose to not only investigate knowledge resistance in a narrow sense, involving a direct, epistemically irrational response to evidence subjects have, but also in a wider sense, resulting for instance from selective exposure.