  According to standard textbooks, the nuclear symmetry energy originates from
the {\it kinetic} energy and the {\it interaction} itself.
  We argue that this view requires certain modifications.
  We ascribe the physical origin of the {\it kinetic} term to the discreteness
of fermionic levels of, in principle arbitrary binary fermionic systems, and
relate its mean value directly to the average level density. Physically it
connects this part also to the isoscalar part of the interaction which, at
least in self-bound systems like atomic nuclei, decides upon the spatial
dimensions of the system. For the general case of binary fermionic systems
possible external confining potentials as well as specific boundary conditions
will contribute to this part. The reliability of this concept is tested using
self-consistent Skyrme Hartree-Fock calculations.
