How do we experience time? What do we use to experience it? In a series of remarkable experiments, Robert Ornstein shows that it is difficult to maintain an "inner clock" explanation of the experience of time and postulates a cognitive, information-processing approach. This approach alone makes sense out of the very different data of the experience of time and in particular of the experience of duration -- the lengthening of the sensation of duration under LSD, for example, or the effects of an experience felt to be a success rather than a failure, time in sensory deprivation, the time-order effect, or the influence of the administration of a sedative or stimulant drug.