  The Feshbach-type reduction of the Hilbert space to the physically most
relevant "model" subspace is suggested as a means of a formal unification of
the standard quantum mechanics with its recently proposed PT symmetric
modification. The resulting "effective" Hamiltonians H(eff) are always
Hermitian, and the two alternative forms of their energy-dependence are
interpreted as a certain dynamical nonlinearity, responsible for the repulsion
and/or attraction of the levels in the Hermitian and/or PT symmetric cases,
respectively. The spontaneous PT symmetry breaking is then reflected by the
loss of the Hermiticity of H(eff) while the pseudo-unitary evolution law
persists in the unreduced Hilbert space.
