The article examines the correspondence (1701) between the Neapolitan mathematician Giacinto De Cristofaro and Domennico Guglielmini, professor of theoretical medicine at Padua, on the role of blood and on the fermentative process in the 'origin' of life. The discussion is set against the background of the lively Italian medical debates and experimentations on the function and composition of the blood. Works by Marcello Malpighi, Giacomo Sandri, Giovanni Maria Lancisi, Giorgio Baglivi, Giovanbattista Morgagni and Nicola Cirillo - all of them taking into account Robert Boyle's Natural History of the Blood (1683-4) - show that blood composition and heart motion were described in different ways, ranging from the adoption of chemical theories and experimentation to that of strictly mechanical explanations. Different positions about the role of the blood and of its fermentative motion reflect different views about the relationship between matter and life.