We propose that the initial state of the Universe was an isotropic state of maximal entropy. Such a state can be described in terms of closed, interacting, fundamental strings in their high-temperature Hagedorn phase and cannot be described by a semiclassical spacetime geometry. If one nevertheless insists on an effective semiclassical description by ignoring the entropy, this leads to a partially equivalent description in which the pressure appears to be negative and equal in magnitude to the energy density, as if the energy-momentum tensor was that of a cosmological constant. From this effective perspective, the state describes a period of string-scale inflation. The stringy microscopic description determines the parameters of the model of inflation, as well as the cosmological observables, in terms of the string length scale and coupling strength.