Predictive processing (PP) has been repeatedly presented as a unificatory account of perception, action, and cognition. In this paper, we argue that this is premature: As a unifying theory, PP fails to deliver general, simple, homogeneous, and systematic explanations. By examining its current trajectory of development, we conclude that PP remains only loosely connected both to its computational framework and to its hypothetical biological underpinnings, which makes its fundamentals unclear. Instead of offering explanations that refer to the same set of principles, we observe systematic equivocations in PP‐based models, or outright contradictions with its avowed principles. To make matters worse, PP‐based models are seldom empirically validated, and they are frequently offered as mere just‐so stories. The large number of PP‐based models is thus not evidence of theoretical progress in unifying perception, action, and cognition. On the contrary, we maintain that the gap between theory and its biological and computational bases contributes to the arrested development of PP as a unificatory theory. Thus, we urge the defenders of PP to focus on its critical problems instead of offering mere re‐descriptions of known phenomena, and to validate their models against possible alternative explanations that stem from different theoretical assumptions. Otherwise, PP will ultimately fail as a unified theory of cognition.

Rubber hand illusion is caused by spatiotemporally congruent visuotactile stimulation which induces a sense of ownership towards a fake limb. We tested two predictions of the Bayesian bottom-up model; namely, that the strength of the illusion is inversely proportional to (a) the distance separating hands and (b) the precision of proprioceptive signals. To manipulate distance, we displaced participants’ hands to either a position close to (8 cm) or far from (24 cm) the rubber hand. Before manipulation, we assessed proprioceptive abilities in a task requiring active reproduction of one’s arm’s position. Proprioceptive precision was operationalised as inversely related to the variance of the estimations. Multiple regression showed that both for subjective and physiological measures neither distance, nor proprioceptive precision, nor their interaction were predictors of illusion strength. Bayes factor analyses provided evidence for null effects. Our findings suggest the limited relevance of proprioception for the strength of visuo-haptically induced rubber hand illusion.