Fermat described his method of infinite descent and gave an example on how it could be used to prove that every prime of the form 4k + 1 could be written as the sum of two squares.
Fermat inilarawan sa kanyang paraan ng walang hanggan pinagbuhatan at ibinigay ng isang halimbawa kung paano ito ay maaaring gamitin upang patunayan na ang bawat kalakasan ng form 4 k + 1 ay maaaring nakasulat na bilang ang kabuuan ng dalawang squares.