5. Other concepts

These notions were highlighted by the study of the functional.

We were forced to define or clarify certain differences between the pure geometry of figures and the imperfect geometry of volumes bounded by surfaces.

A straight line doesn't exist, if not fictitious (absolute axis), nor does a point, if not fictitious (point on an axis).

Only the surface is palpable, and therefore measurable.

So don't be surprised if you don't have any straight lines to tolerate (contrary to what some misguided didactics would like to recommend).