Abstract
While early studies cataloged striking errors of physical scene understanding in naive observers, a resurgence of research has instead revealed a remarkable ability to intuit how physical scenes will unfold. Properties such as stability, mass, gravity and kinematics may all elicit accurate intuitions, especially when presented in naturalistic settings. A leading interpretation of these findings is that physical scene understanding recruits a general-purpose “physics engine”. Under this theory, the mind simulates how a scene will unfold by modeling or approximating its physics, and only fails for physics problems that are contrived or presented without context. But might there be tasks that persistently strain physical reasoning, even when presented naturalistically and in realistic contexts? Here, 5 experiments explore this question by evaluating intuitions about the strength of simple knots. Knots are naturalistic stimuli that are ubiquitous across cultures and time periods, and in many contexts evaluating them correctly can spell the difference between safety and peril. Despite this, here we show that naive observers fail to discern even striking differences in strength between knots. In a series of two-alternative forced-choice tasks, observers viewed simple “bends” (i.e., knots joining two lengths of string) and decided which would require more force to undo. Though the relative strength of these bends is well-documented, observers’ judgments completely failed to reflect these distinctions—including in naturalistic photographs (E1), idealized renderings (E2), dynamic videos (E3), and even when accompanied by schematic diagrams of the knots’ structures (E4). Moreover, these failures persisted despite observers demonstrating visual understanding of the topological differences between the knots (E5); in other words, even when observers knew exactly what kind of knot they were viewing, they failed to extract or predict its physical behavior. These results expose a blindspot in physical reasoning, posing a new kind of challenge to general-purpose theories of scene understanding.