  It was repeatedly underlined in literature that quantum mechanics cannot be
considered a closed theory if the Born Rule is postulated rather than derived
from the first principles. In this work the Born Rule is derived from the
time-reversal symmetry of quantum equations of motion. The derivation is based
on a simple functional equation that takes into account properties of
probability, as well as the linearity and time-reversal symmetry of quantum
equations of motion. The derivation presented in this work also allows to
determine certain limits to applicability of the Born Rule.
