where each event is characterized by its displacement vector where
Then the abscissa of event is defined by
where is arbitrary and often set to zero. The ordinate is defined as
And the -cooordinate or applicate of event is
The three numbers , and are called the Cartesian coordinates of event after the work of René Descartes . More exactly, they are the rectangular Cartesian coordinates in a descriptive system that is centered on P. We use them to express the position of an event as
EthnoPhysics uses a finite categorical scheme of binary distinctions to describe sensation. So , the total number of quarks in a description, may be large but not infinite. In principle is finite and accordingly displacements may be small, negligible or nil, but not infinitesimal. Later we assume that is large enough to make an approximation to spatial continuity. Then the use of calculus may be appropriate.