Mechanisms, microtubules and the mind

The following is an edited version of Roger Penrose's lecture at the Fifth Mind and Brain Symposium at the Institute of Psychiatry, London, on 29 October 1994, introducing the themes of his recent book Shadows of the Mind. The talk begins by outlining some options for the modelling of the relationship between consciousness and computation, and provides evidence for a model in which it is not possible even in principle to simulate mathematical understanding computationally. It is argued that mathematical understanding is on a continuum with consciousness in general, and that non-computability is a feature of all consciousness. The talk then goes on to outline some of the problems of the relationship between quantum and classical physics and proposes a new theory of ‘objective reduction’ by quantum gravity to bridge the explanatory gap. The talk concludes by examining cytoskeletal microtubules as a possible site for quantum-coherent events in the brain. It is suggested that this might be the physical basis of conscious events.