This article aims to clarify the epistemological foundations of the Freudian energetics model, starting with a historical review of the 19th century scientific context in which Freud's research lay down its roots. Beyond the physiological and anatomical references of Project for a Scientific Psychology (Freud, 1895a), the physiology Freud makes reference to is in reality primarily anchored in an epistemological model derived from physics. Whilst across the Rhine, the autonomy of physiology in relation to physics was far from being accomplished, as a counterpoint, in France, the revolution in physiology driven by Claude Bernard established itself autonomously from physics,. In contrast, Freud's scientific landscape is entirely dominated by the physics elevated to the rank of an ideal science. The influence of Helmholtz, who is both a medical doctor and a physicist, has a determining influence on Freud's training. The discoveries in physics at that time, in particular the formulation of the principle of 'conservation of force' first principle of thermodynamics - will constitute the points of reference upon which Freud will elaborate his energetics model, then subsequently, the idea of economy in his metapsychology. In this way we can trace both the historic and epistemological path that led Freud from a concept based on physics, and more specifically thermodynamic energy, to an idea of nervous energy that constitutes the basis of the concept of "quantity" as it is stated as 'first fundamental idea' in Project for a Scientific Psychology (Freud, 1895a). This notion will subsequently evolve, and lead Freud to the introduction of the concept of 'psychical energy,' this time in a purely metapsychological sense.