Why did critics argue that Descartes had not defeated skepticism?

Critics argued that René Descartes failed to defeat skepticism because his attempt to establish a foundation for knowledge was inherently circular, a problem famously identified by Antoine Arnauld as the Cartesian Circle.12

Descartes sought to prove the existence of God and the external world by relying on the criterion that whatever he perceived "clearly and distinctly" must be true. However, he argued that the reliability of these clear and distinct perceptions was guaranteed only by the existence of a perfect, non-deceiving God. Consequently, his reasoning became circular: he used clear and distinct perceptions to prove that God exists, while simultaneously relying on the existence of God to validate the truth of those clear and distinct perceptions.132

Throughout the 17th century, other skeptical critics—including Marin Mersenne, Pierre Gassendi, and Pierre-Daniel Huet—challenged the validity of the cogito and the application of his method. They contended that if Descartes had sincerely followed his own skeptical procedure, his system would have inevitably led to complete skepticism rather than the certainty he claimed to have achieved.4