Several authors have hypothesized that completely positive maps should provide the means for generalizing quantum dynamics. In a critical analysis of that proposal, we show that such maps are incompatible with the standard phenomenological theory of spin relaxation and that the theoretical argument which has been offered as justification for the hypothesis is fallacious.

The assertion by Simmons and Park that the dynamical map associated with the Bloch equations of nuclear magnetic resonance is not completely positive is wrong.