   It is impossible to know exactly why Copernicus began to espouse the heliocentric cosmology. Despite his importance in the history of philosophy, there is a paucity of primary sources on Copernicus. His only astronomical writings were the Commentariolus, the Letter against Werner, and On the Revolutions; he published his translation of Theophylactus’s letters and wrote the various versions of his treatise on coinage; other writings relate to diocesan business, as do most of the few letters that survive. Sadly, the biography by Rheticus, which should have provided scholars with an enormous amount of information, has been lost.  Therefore, many of the answers to the most interesting questions about Copernicus’s ideas and works have been the result of conjecture and inference, and we can only guess why Copernicus adopted the heliocentric system.