The author proposes that the anonymous letter dated May 19th 1641 and delivered to Descartes by Mersenne should be attributed to Thomas Hobbes. Although the content is known, what scholars are usually more interested in are Descartes’ two replies, which contain important clarifications on the proof of God’s existence. That the letter was written by Hobbes is revealed by various thematic, conceptual, and lexical analogies and, above all, by the presence of two doctrines characteristic of his thought: 1) the denial of the existence of intellectual ideas; 2) the assertion that the nature of God can only be described by the proposition "God exists". Attributing this letter to Hobbes throws new light on the debate that followed Descartes’ Meditations as well as on the role played by Mersenne.
Keywords: Hobbes, Descartes, Mersenne, idea, imagination, intellect