  We consider the category of Harish-Chandra modules for ${\rm SL}_2(\mathbb
R)$ as a module over the category of finite-dimensional representations of
${\rm SL}(2)$ with respect to the tensor product. In this note we use classical
results about principal series to obtain a classification of
$\otimes$-submodules of the category of Harish-Chandra modules of ${\rm
SL}_2(\mathbb R)$. The resulting classification recovers the classical
classification of irreducible Harish-Chandra modules. Our methods are expected
to generalize to arbitrary reductive pairs and more general base fields.
