Heuristics and invariants in dynamic event perception: Immunized concepts or nonstatements?

There is ample evidence showing that observers are able to judge dynamic properties of moving objects on the basis of visual kinematics. It is, however, widely disputed as to how to explain this body of data. While direct perceptionists claim that observers have direct access to the underlying dynamics of the event (kinematic specification of dynamics, or KSD, principle), contenders of the perceptual heuristics approach (PH) suggest that observers use the visual equivalent of a heuristic to arrive at their judgments about dynamics. First, the critical assumptions of the KSD principle are discussed in order to motivate the claim that the KSD principle cannot be falsified and is thus immune to empirical criticism. Then the same scrutiny is applied to the PH approach. It is, in its general form at least, afflicted with a similar lack of falsifiability. In comparing the concepts underlying invariants and perceptual heuristics, my conclusion is that a critical experiment aimed at deciding between the two approaches is impossible, and that KSD and PH can be regarded as nonstatements. The findings reported in the event-perception literature must therefore be reevaluated on these grounds.