Return to lower phases: concrete and abstract.—Formation of idea of unity. Hypotheses as to its experimental origin: touch, sight, hearing, internal sensations, attention. Unity the result of decomposition, an abstract.—The series of numbers. Process of construction.—Function of signs: discussions of this subject 137 _Section II.—The Concept of Space._ Extension as a concrete fact. Variable and relative characteristics.—Transition to concrete-abstract period.—Space (abstract): the current popular conception the result of imagination. Idle problems.—The true concept is the result of dissociation.—The notion of “function.”—Imagination of an infinite space.—Works on ideal geometry: constructive power of the mind; reinforcement of distinction between space as perceived and conceived 146 _Section III.—The Concept of Time._ Real (concrete) duration: the present, its reality; its experimental determination: maximum and minimum. Reproduction of duration; experiments; indifferent point.—Variable and relative characteristics.—Origin of concrete notion of time: different hypotheses: external and internal sensations: presumption in favor of the latter.—Abstract duration (time). First stage, depends on memory and imagination only: corresponds (1) to generic images (representation of duration among the higher animals), and (2) to the concrete-abstract period (intermediate forms of abstraction). Primitive races. Why has time (and not space) been personified.—Second stage depends upon abstraction. Function of the astronomers: measure of time.—Infinite time.—Current hypotheses as to the psychologic process which constitutes the notion of time: sensations and consecutive images: sensations which are feelings of tension, of effort. “Temporal signs.”—Full and empty time 159 _Section IV.—The Concept of Cause._