In a letter to Mersenne of April 15, 1630, Descartes announces his first metaphysical thesis, known as the doctrine of free creation of eternal truths. With this doctrine, Descartes breaks with a long tradition and sustains that God creates not only the existences, but also the essences of things. Although this unorthodox thesis runs through his work, Descartes never gives it a systematic explanation. However, its main interpreters recognize its extreme originality, difficulty and importance for determining the exact meaning of Descartes’ project of grounding metaphysically the human science. The article analyzes some central elements of this thesis in order to clarify its philosophical justification and its consequences for the understanding of the ontological, logical and epistemological status of the necessary truths through which our finite intellect intends to ground an absolutely certain science.