Hallucination lies at an intriguing border between psychiatry and philosophy. Although Behrendt & Young (B&Y) tie their proposal to Kantian transcendental idealism, other philosophical positions are equally consistent. Cognition is underconstrained by reality not only in hallucination but also in autism and dreaming. Sensory underconstraint is insufficient to encompass schizophrenia. There is also a breakdown in integrative capacity on the cognitive side. From a wider clinical perspective than schizophrenia, there can be underconstraint or overconstraint in sensory and cognitive functionalities.