96 SPACE AND GEOMETRY
of more or less distinguishable sensational qualities, conjoined with adjacent distinguishable, continuously graduated locative qualities. If such objects move, particularly in the domain of our hands, we perceive them to shrink or swell (in whole or in part), or we perceive them to remain the same; in other words, the contrasts characterizing their bounding locative qualities change or remain constant. In the latter case, we call the objects rigid. By the recognition of permanency as coincident with spatial displacement, the various constituents o>f our intuition of space are rendered comparable with one another,—at first in the physiological sense. By the comparison of different bodies with one another, by the introduction of physical measures, this comparability is rendered quantitative and more exact, and so transcends the limitations of individuality. Thus, in the place of an individual and non-transmittable intuition of space are substituted the universal concepts of geometry, which hold good for all men. Each person has his own individual intuitive space; geometric space is common to all. Between the space of intuition and metric space, which contains physical experiences, we must distinguish sharply.
RIEMANN'S PHYSICAL CONCEPTION OF GEOMETRY.
The need of a thoroughgoing epistemological elucidation of the foundations of geometry induced Riemann,1 about the middle of the century just
1Ue'ber die Hypothesen, welche der Geometric zu Grunde liegen. Gottingen, 1867.