For example, although he wrote to Fermat praising his work on determining the tangent to a cycloid (which is indeed correct), Descartes wrote to Mersenne claiming that it was incorrect and saying that Fermat was inadequate as a mathematician and a thinker.
Halimbawa, kahit na siya wrote Fermat pagpuri sa kanyang trabaho sa pagtukoy sa mga padaplis sa isang saykloyd (na sa katunayan ay tama), Descartes wrote Mersenne sa pagtubos na ito ay hindi tama at sinasabi na Fermat ay kapos bilang isang dalub-agbilang at isang palaisip.