  We show that the reasoning in favor of a symmetric couple stress tensor in
Yang et al.'s introduction of the modified couple stress theory contains a gap,
but we present a reasonable physical hypothesis, implying that the couple
stress tensor is traceless and may be symmetric anyway. To this aim, the origin
of couple stress is discussed on the basis of certain properties of the total
stress itself. In contrast to classical continuum mechanics, the balance of
linear momentum and the balance of angular momentum are formulated at an
infinitesimal cube considering the total stress as linear and quadratic
approximation of a spatial Taylor series expansion.
