  We study the problem of discriminating between non-orthogonal quantum states
with least probability of error. We demonstrate that this problem can be
simplified if we solve for the error itself rather than solving directly for
the optimal measurement. This method enables us to derive solutions directly
and thus make definite statements about the uniqueness of an optimal strategy.
This approach immediately leads us to a state-discrimination analogue of Davies
Theorem.
  In the course of this, a complete solution for distinguishing equally likely
pure qubit states is presented.
