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One of the most interesting differences between the pre-big bang scenario and the standard cosmological models is in the description of the initial state of the universe. Contrary to the basic paradigm of modern cosmology, in the pre-big bang model the universe is initially in a classical state, the most general perturbative solution of the tree-level low-energy effective string action. The authors discuss here the naturaliness of this initial state and the conditions that the initial classical fluctuations should satisfy in order to generate an inflationary phase. The authors argue that the occurrence of a period of inflation in this scenario is as generic in string theory as that of gravitational collapse in general relativity.