The Euler–Cauchy stress principle states that upon any surface (real or imaginary) that divides the body, the action of one part of the body on the other is equivalent (equipollent) to the system of distributed forces and couples on the surface dividing the body,[1] and it is represented by a field T ( n ) {\displaystyle \mathbf {T} ^{(\mathbf {n} )}} , called the stress vector, defined on the surface S {\displaystyle S} and assumed to depend continuously on the surface's unit vector n {\displaystyle \mathbf {n} } .[2][3]:p.66–96