Our space is called three-dimensional because it takes three numbers--measurement in three mutually perpendicular directions--to determine and mark out any particular point from the totality of points. Time, as the individual experiences it, is called one-dimensional for an analogous reason: one number is all that is required to determine and mark out any particular event of a series from all the rest. Now in order to establish a position in a space of four dimensions it would be necessary to measure in _four_ mutually perpendicular directions. Time curvature opens up the possibility of a corresponding higher development in time: one whereby time would be more fittingly symbolized by a plane than by a linear figure. Indeed, the familiar mystery of memory calls for such a conception. Memory is a carrying forward of the past into the present, and the fact that we can recall a past event without mentally rehearsing all the intermediate happenings in inverse order, shows that in the time aspect of memory there is simultaneity as well as sequence--time ceases to be linear and becomes _plane_. More remarkable illustrations of the sublimation of the time-sense are to be found in the phenomena of sleep and dreams. "Oh, thou that sleepest, what is sleep?" asks the curious Leonardo. Modern psychological science has little to offer of a positive nature in answer to this world-old question, but it has at least effectively disposed of the absurd theories of the materialists who would have us believe that sleep is a mere matter of blood circulation or of intoxication by accumulation of waste products in the system. Sleep states are not abnormal, but part and parcel of the life existence of the individual. When a person is asleep he has only become unresponsive to the mass of stimuli of the external world which constitutes his environment. As Sidis says, "When our interest in external existence fags and fades away, we go to sleep. When our interests in the external world cease, we draw up the bridges, so to say, interrupt all external communication as far as possible, and become isolated in our own fortress and repair to our own world of organic activity and inner dream life. Sleep is the interruption of our intercourse with the external world: it is the laying down of our arms in the struggle of life. Sleep is a truce with the world." The twin concepts of higher space and curved time sanction a view of sleep even bolder. Sleep is more than a longing of the body to be free of the flame which consumes it: the flame itself aspires to be free--that is to say, consciousness, tiring of its tool, the brain, and of the world, its workshop, takes a turn into the plaisance of the fourth dimension, where time and space are less rigid to resist the fulfillment of desire. DREAMS