  In a recent paper [Phys.~Rev.~A {\bf 91}, 053844 (2015)], Mukamel and Dorfman
compare spectroscopies performed with classical vs.~quantum light, and conclude
that \textit{nonlinear} quantum-spectroscopy signals cannot be obtained from
averaging their classical-spectroscopy counterparts over the Glauber--Sudarshan
quasiprobability distribution of the quantum field. In this Comment, we show
that this interpretation is correct only if one assumes that a classical signal
is given by a classical approximation for the field. While such an assumption
can be useful for comparing theoretical results, it is never realized in laser
spectroscopy experiments that typically use coherent states. Thus, instead of
using classical signals, the connection between coherent states and quantum
states of light must be considered. We rigorously show that quantum
spectroscopy can always be projected from the experimentally realized
coherent-state spectroscopy regardless how nonlinear the system response is.
