When the Ideologues referred to animals, they seemed to hesitate between a conception of the “l’animal-sensible”, associated to Condillac, and a theory of the “l’animal-machine” traceable to Descartes. To understand this paradox, this article aims to clarify the position of the Ideologues by the notion of “naturalism” such as it is defined by Philippe Descola. The first part of the article shows that Cabanis, Destutt de Tracy, Roederer et Jean-Baptiste Say sought to renew the conception of the “l’animal-machine” in light of the chemistry of Lavoisier. Their concern of putting the former in the service of a republic of small property owners distinguishes them from the approach of Chaptal, when he was minister of the Interior. The second part of the article demonstrates that the Ideologues deepened the Condillacien theory of “animal-sensible.” The project of “comparative ideology” of the naturalist Montpellier Draparnaud illustrates their concern about an animal ethics.