Abstract: Title: Is Perception Probabilistic? Scientific question Is perception probabilistic? This is a grand question about the fundamental nature of how our mind operates and how this operation is implemented in brain circuits. The question is also interpreted differently by different people, making progress difficult. In this adversarial collaboration, four scien- tists with different backgrounds and perspectives on this issue1–7 engaged in considerable discussion of how the issue of probabilistic perception can be operationalized as to be made empirically tracta- ble. We have agreed that a critical dividing line between probabilistic and non-probabilistic views of perception is defined by the following question: Can humans perform flexible and deliberate (i.e., cognitive) computations that require access to more than summary statistics (mean and variance) of a perceptual likelihood function or posterior on a single-trial basis?