As suggested by Ricoeur (1965) the exact sciences have often provided the language by means of which Freud articulated the psychic apparatus, although the articulation itself is irreducible to the physico-physiological level. It is well known that in Jenseits des Lustprinzips (1920g), Freud stressed the need to “borrow words from biological science” and to use them as a “metaphorical language” in the description of psychological phenomena. This paper investigates the way in which the specific physicalist semantics of one such borrowed word, namely the concept of “(psychic) energy” conditioned Freud’s metapsychological formalization of the psyche. The general framework for this study is constituted by the historical conflict of interpretations (and more specifically by the antinomy between mechanical-causal and teleological interpretation) in the understanding of man and world. Through a sketch of the Vis Viva debate between Leibniz and Descartes at the beginning of modernity, two conceptual schemes pertaining to the concept of energy (as Bewegungskraft on the one hand, and dynamic essence on the other) will be made explicit, and interpreted along the lines of the historical tension between quantitative-physical and quantitative-metaphysical explanations. Consequently, it is demonstrated how the final affirmation of energy as kinesis in the 20th century influenced Freud in his theorizing, and impeded the explicit articulation of the subject as an intentional structure.