▁his ▁idea ▁that ▁the ▁earth ▁is ▁flat ▁had ▁been ▁a ▁minor ity ▁view ▁among ▁educated ▁Western ▁opinion ▁since ▁the ▁ 3 rd ▁century ▁BC . ▁Cos mas ' s ▁view ▁was ▁never ▁influ ential ▁even ▁in ▁religious ▁circles ; ▁a ▁near - cont empor ary ▁Christian , ▁John ▁Phil op on us , ▁disag re ed ▁with ▁him ▁as ▁did ▁many ▁Christian ▁philosoph ers ▁of ▁the ▁era . ▁ ▁David ▁C . ▁Lind berg ▁as ser ts : ▁" C os mas ▁was ▁not ▁particularly ▁influ ential ▁in ▁By z ant ium , ▁but ▁he ▁is ▁important ▁for ▁us ▁because ▁he ▁has ▁been ▁commonly ▁used ▁to ▁but tr ess ▁the ▁claim ▁that ▁all ▁( or ▁most ) ▁medieval ▁people ▁believed ▁they ▁lived ▁on ▁a ▁flat ▁earth . ▁This ▁claim ... is ▁totally ▁false . ▁Cos mas ▁is , ▁in ▁fact , ▁the ▁only ▁medieval ▁European ▁known ▁to ▁have ▁def ended ▁a ▁flat ▁earth ▁cosm ology , ▁whereas ▁it ▁is ▁safe ▁to ▁assume ▁that ▁all ▁educated ▁Western ▁Europe ans ▁( and ▁almost ▁one ▁hundred ▁percent ▁of ▁educated ▁By z ant ines ), ▁as ▁well ▁as ▁sail ors ▁and ▁travel ers , ▁believed ▁in ▁the ▁earth ' s ▁spher icity ." ▁ ▁Cos m ology ▁aside , ▁Cos mas ▁proves ▁to ▁be ▁an ▁interesting ▁and ▁reliable ▁guide , ▁providing ▁a ▁window ▁into ▁a ▁world ▁that ▁has ▁since ▁disappeared . ▁He ▁happened ▁to ▁be ▁in ▁Ad ul is ▁on ▁the ▁Red ▁Sea ▁Coast ▁of ▁modern ▁E rit rea ▁at ▁the ▁time ▁( c . ▁ 5