As Freud set out as early as in the ‘Project for a Scientific Psychology’ (1895), ‘The intention is to furnish a psychology that shall be a natural science’. The aim of this theoretical, interdisciplinary article is to bridge classical psychoanalysis and thermodynamics. The Einstein model of a Solid (ES) is a thermodynamic model with applications in solid-state physics. I intend to show that the ES, in the grand canonical or the canonical ensemble, is a suitable model of the mental apparatus from the economic point of view. The approach I follow is to establish a strict one-to-one correspondence between the constituents of the ES, quantum harmonic oscillators (QHOs), and energy quanta, and those of the psychic apparatus, ideas, and quota of affect, respectively. I also prove that the laws of thermodynamics, as applied to the ES, are correlated with the principles of the economic model, Pleasure, Reality, Nirvana, and Constancy. In case we consider the Ucs. as a sealed-off chamber, impermeable to ideas to the ego, then the proper ensemble is the canonical ensemble. In this case, the average energy of the ego remains the same, but the fluctuations in the number of particles (QHOs) disappear. The significance of this research is in introducing science into psychoanalytic theory.