The Ch.W. Hufeland's concept of the organism as the seat of vital force, presented in his best known work entitled 'Die Kunst das menschliche Leben zu verlängern' (1797) is discussed in detail. In accordance with the conditions in which the vital force operates (or: in which the property of animateness occurs), it can assume the following three forms: (a) Organic force, which integrates the building blocks of the body and maintains the cohesion between them. (b) Plastic force, which subjects to transformations, taking place over time, a whole that emerges in an organic way, thus ultimately endowing it with a specific form. (c) The force-faculty of responding to stimuli (Reizfähigkeit), which could be termed excitability in the broadest sense. The single vital force acts simultaneously in all its three varieties, i.e. at the same time it annuls chemical laws, it controls the morphogenetic process, and it also responds specifically to internal and external stimuli. It is worth adding that, from time to time, Hufeland felt sceptical with regard to that unknown cause-force: he doubted whether it would ever be possible to know it, but more frequently he took that lack of knowledge to be a temporary, transitional state. Indeed, he described this force in conditional terms as 'hitherto unknown' or 'as yet not known'. Hufeland sent his treatise to Kant, for whom the treatise became an inspiration for a new, very peculiar dissertation, incorporated as a third part into a larger work that he published: 'Der Streit der Fakultäten', and entitled it 'Der Streit der philosophischen Fakultät mit der medicinischen'. Luckily, letters between Hufeland and Kant relating to the publication of that dissertation have been preserved.They provide some information on the last years of the philosopher's life. These letters, together with two fragments from the first edition of Kant's dissertation are presented in Polish translation for the first time in the current paper.