By means of the analysis of three works (Dell'anima de' bruti [Of the soul of beasts], Sofilo Molossio, and Sofilo senza maschera [Sofilo without a mask]) of Alessandro Pascoli (1669-1757), the psysician and philosopher from Perugia, the article reconstructs his fluctuating thought with regard to the problem of sensation in animals, indicated as the problem of the "soul of beasts." Regarding this question, Pascoli oscillates between, on the one hand, the Cartesian theory, which considered animals similar to mechanical automatons, devoid of the capacity to experience sensations (that is say, devoid of "sensitivity"); and, on the other hand, the Church's scholastic-peripatetic doctrine that attributed to animals the capacity to feel, thus affirming the presence in them of a "sensitive soul," considered -as compared with the human one -imperfect, material, and mortal. In expounding the reasons and argumentations of the Cartesians, on the one hand, and of the ecclesiastic teachings, on the other, Pascoli manifests a substantial convergence with the former, but also the need, inasmuch as Catholic professor of medicine at the Sapienza University of Rome, to not deny the possibility of the latter. In this tormented and contorted alternation of opinions, between the thesis of the animal-machine and that of the animal gifted with a sensitive soul, he introduces conceptual elements that, further developed, will end up by conducting to the ideas of "vital property" and of "vital principle" typical of the vitalistic thought of the 18th and 19th centuries.