Hope is carefully to be distinguished, on the one hand from optimism (which springs from prediction of what the future will bring), and on the other hand from wishful thinking (which is unconstrained by the probabilities of what that future might bring). Hope is based neither on certainty, as if it were simply extrapolation of the present, nor on fantasy, as if its object bore only a tenuous relation to the present. Once again, we encounter the eschatological dialectic of continuity and discontinuity. In relation to hope, failure to respect this balance can lead either to despair that anything will ever change for the better, or to violent imaginings of apocalyptic destruction in which the future can be attained only by the annihilation of the past.
JOHN POLKINGHORNE, The God of Hope and the End of the World