In this contributionMathematical we deal with an aspect that at first could not seemItaly directly related with the discussions by Lord Kelvin and his colleagues on the fundamentals of the theory of heat. Indeed, we focus on William Thomson’s profound spiritualism, which led him to found all of his natural philosophyPhilosophy on the immutability of some quantity, as established by the Creator himself. After his stay in ParisParis in 1845, where he worked at the laboratory of Henri-Victor Regnault, Lord Kelvin strongly supported the French scholars (e.g., Carnot and Clapeyron) who investigated thermal phenomenaPhenomena by introducing the caloric fluid. According to them, just like water produces mechanical power when falling through a difference in height, so does caloric when falling through a difference of temperatures. That is, caloric contains vis vivaVis viva, a known, established concept in the mechanics of solid and fluid bodies. This entity must be conserved, since it is not possible to imagine a human processProcess that can produce perpetual motionPerpetual motion. Thus, a fundamental postulate of William Thomson’s investigations on natural philosophy is that the Creator is the only supreme being that can create or annihilate vis vivaVis viva. This originated a deep debate for at least two decades, in which Lord Kelvin investigated deeply on the possibility that heat can transform itself into vis vivaVis viva and vice versa. We will analyse in some detail this conception and these investigations, with reference to their historical setting, providing some background of the English and European debate on the subject of the time.