In Diderot’s philosophy, the nature of the eighteenth-century, Isis veiled, is constituted of the same substance as the metaphor, the analogy and the hieroglyph. To show that, this article takes into consideration the naturalistic inquiry on Trembley’s Hydra. This animal, which is at the heart of the philosophical interest of the period, seems to shape itself starting from the mythological imagination, but at the same time it becomes the model that, for Diderot, defines the faculty of thinking and its features. (...) The reversibility between word and image, nature and culture, representation and the represented thing provides the observation point on a philosophy – Diderot’s philosophy – that develops itself along with the human physiology and imagination, its own faculty. (shrink)