The three spatial characteristics of length, height and depth are used in the same unreflective way by laymen, technicians and scientists alike to describe the forms, positions and measure of bodies and hollow bodies. But how do we know that the space we live in has just three dimensions? This question has occupied philosophers and scientists since antiquity. The answers proposed have become ever more presumptuous and have increasingly lost sight of everyday intuitions and have sacrificed explanatory power. In this work, Peter Janich shows that all explanations of three-dimensionality hinge on an unreflective geometrical language which seems to accept the lack of an alternative for the three sorts of entities - lines, planes and solids. This is a Euclidean heritage in a dual sense. Euclid himself adopted a geometrical language from the art of figure drawing, and left a tradition of doing geometry as planimetry and of doing steremetry by rotating plane figures. The systematic approach offered here starts out from operational definitions of the spatial forms - plane, straight edge and perpendicularity - and proofs that only three planes can intersect pairwise orthogonally. This is the constructive solution in the frame theory of action, providing an unequivocal characterization of spatial relations in the physical world. The traditional order of geometric concepts turns out to be the most important obstacle to the methodical ordering of everyday scientific concepts.