In a recent article G. Hatfield claims that Descartes for a certain time thought a purely a priori science to be possible. Hatfield's evidence consists of his reading of the Cartesian method in the Regulae and of a letter to Mersenne, written in May 1632. I argue that Hatfield misinterprets the Cartesian method and Descartes' claim in the letter to Mersenne. I first show that the latter does not argue for an a priori science. Then, I show that the method of the Regulae is not a priori. Finally, I propose a reading of Descartes' letter.