  We consider the Euclidean path integral approach to higher-derivative
theories proposed by Hawking and Hertog (Phys. Rev. D65 (2002), 103515). The
Pais-Uhlenbeck oscillator is studied in some detail. The operator algebra is
reconstructed and the structure of the space of states revealed. It is shown
that the quantum theory results from quantizing the classical complex dynamics
in which the original dynamics is consistently immersed. The field-theoretical
counterpart of Pais-Uhlenbeck oscillator is also considered.
