There is no pure visualization in the sense of a priori philosophies; every visualization is determined by previous sense perceptions, and any separa… - Hans Reichenbach

If heat were the affecting force, direct indications of its presence could be found which would not make use of geometry as an indirect method. ...direct evidence for the presence of heat is based on the fact that it affects different materials in different ways. ...The forces... which we have introduced... have two properties: (a) They affect all materials in the same way. (b) There are no insulating [or isolating] walls. ...the definition of the insulating wall may be added here: it is a covering made of any kind of material which does not act upon the enclosed object with forces having property a. Let us call the forces which have the properties a and b universal forces; all other forces are called differential forces. Then it can be said that differential forces, but not universal forces, are directly demonstrable.

If E₁ is the cause of E₂, then a small variation (a mark) in E₁ is associated with a small variation in E₂, whereas small variations in E₂ are not associated with variations in E₁. If we wish to express even more clearly that this concept does not contain the concept of temporal order, we can express it in the following form, where events that show a slight variation are designated E*: E₁E₂, E₁*E₂*, E₁E₂* and never the combination E₁*E₂.

Light signals alone provide the metrical structure of the four-dimensional space-time continuum. The construction may be called light axioms.