  The orbital angular momenta $L^u$ and $L^d$ of up and down quarks in the
proton are estimated as functions of the energy scale as model-independently as
possible, on the basis of Ji's angular momentum sum rule. This analysis
indicates that $L^u - L^d$ is large and negative even at low energy scale of
nonperturbative QCD, in contrast to Thomas' similar analysis based on the
refined cloudy bag model. We pursuit the origin of this apparent discrepancy
and suggest that it may have a connection with the fundamental question of how
to define quark orbital angular momenta in QCD.
