Classical physics assumed that there were laws according to which the knowledge of the initial state of all the smallest particles enabled us to predict the final state unambiguously. This was called knowledge of the microstate. However only certain average values are accessible to experiment; for example, instead of the position of all individual particles, only the density of matter, the mass per unit volume, whereas the positions of the individual particles themselves cannot be observed. Nor can the velocity of each individual particle be observed, but only the average kinetic energy, the temperature. Density and temperature determine the macrostate of the body. Obviously very many microstates can correspond to one and the same macrostate, since the same average value can result from very different mixtures of individual values; the future is therefore not unambiguously determined by the macrostate.