  It is proposed that $S$-duality is a fundamental symmetry of nature which is
spontaneously broken. Axion and dilaton are identified with the doublet of the
$S$-duality symmetry group $SL(2,\mathbbm{R})$. The symmetry is broken at a
high scale corresponding to the experimentally estimated axion decay constant
$f_{\chi}$. The symmetry breaking mechanism is discussed in analogy with PCAC
in pion physics. $S$-duality invariant interactions of fermions with axion and
dilaton doublet are introduced. The symmetry breaking mechanism contributes
negligibly small corrections to fermion masses in the QCD sector. Inspired by
universality in string theory, the $S$-duality invariant interaction of the
axion-dilaton doublet to QCD fermions is proposed to generalize to all
fermions. Phenomenological consequences of this broken symmetry are explored.
