  The diphoton channel at lepton colliders, $e^+e^- (\mu^+\mu^-) \to \gamma
\gamma$, has a remarkable feature that the leading new physics contribution
comes only from dimension-eight operators. This contribution is subject to a
set of positivity bounds, derived from the fundamental principles of Quantum
Field Theory, such as unitarity, locality, analyticity and Lorentz invariance.
These positivity bounds are thus applicable to the most direct observable --
the diphoton cross section. This unique feature provides a clear, robust, and
unambiguous test of these principles. We estimate the capability of various
future lepton colliders in probing the dimension-eight operators and testing
the positivity bounds in this channel. We show that positivity bounds can lift
certain flat directions among the effective operators and significantly change
the perspectives of a global analysis. We also discuss the positivity bounds of
the $Z\gamma/ZZ$ processes which are related to the $\gamma\gamma$ ones, but
are more complicated due to the massive $Z$ boson.
