Contents: A Language of Space; Four dimensional space; Analogy of a plane world; Significance of a four-dimensional existence; First chapter in the history of four space; The higher world; Evidences for a fourth dimension; Use of four dimensions in thought; Application to Kant's theory of experience; A four-dimensional figure; Nomenclature and analogies; Simplest four-dimensional solid; Remarks on the figures; A recapitulation and extension of the physical argument.