All four of the previously discussed groups of approaches, the purely spatial, the kinematic, the dynamic and the causalistic, attempt to provide a basis for the three-dimensionality of space — at least to the extent that they do not belong to those rather ridiculous groups of approaches which play around with numerical speculations — through reference to properties of the outer world, of bodies and their motions, to forces in effect between bodies, or the causality which is found in the world of bodies. The fifth and last group, which shall now be discussed, does not search for reasons in the objects of human knowledge, but in the subject which gains knowledge itself. In this, such things are considered as the construction of our sensory organs, the biologically determined structure of our sensory perception, the construction of our organs of equilibrium, the interaction between perception and the motion of the perceiving subject and, finally, the thesis of evolutionary adaption of organisms to their environment, which can be recognized as a three-dimensional environment with the capabilities of organs selected through natural history.