  In this note we generalize the well-known Wigner's unitary-anti\-unitary
theorem. For $X$ and $Y$ smooth normed spaces and $f:X\to Y$ a surjective
mapping such that $|[f(x),f(y)]|=|[x,y]|$, $x,y\in X$, where $[\cdot,\cdot]$ is
the unique semi-inner product, we show that $f$ is phase equivalent to either a
linear or an anti-linear surjective isometry. When $X$ and $Y$ are smooth real
normed spaces and $Y$ strictly convex, we show that Wigner's theorem is
equivalent to $\{\|f(x)+f(y)\|,\|f(x)-f(y)\|\}=\{\|x+y\|,\|x-y\|\}$, $x,y\in
X$.
