The theory developed by Onsager and Machlup was meant to quantify the probability of fluctuation paths as they evolve in time, rather than the mere statistics of fluctuation values. It is a theory based on stochastic perturbations of deterministic dynamics that describe the irreversible relaxation to equilibrium of the thermodynamic observables of macroscopic objects when they are moderately shifted from their equilibrium states. The deterministic contribution represents the irreversible thermodynamic behavior, while the stochastic contribution represents the molecular chaos, assumed to be generated by time reversal invariant Hamiltonian dynamics.