ponder: implies a careful weighing of a problem, and a prolonged inconclusive thinking about a matter: ~ at length the various recourses open to him. (recourse: 依赖，求助  )


meditate: implies a definite focusing one's thoughts on something so as to understand it deeply: the sight of the ruins prompted him to ~ on human vanity. (vanity虚荣)


muse: suggests a more or less focused and persistent but languid and inconclusive turning over in the mind as if in a dream, fancy, or remembrance: ~ upon the adventures of the heroines  of Gothic novels. (languid: 慢悠悠的a languid afternoon懒洋洋的下午)


ruminate: implies going over the same matter in one's thoughts again and again, but does not say anything about purposeful thinking or rapt absorption: the product of 50 yrs of ~ on the meaning of life.


ponder: 仔细的衡量、久久不定的思考问题,  meditate: 专注思考以深入理解,  muse: 慢悠悠、无结果的回想(turn over in the mind)如梦幻幻想或回忆，ruminate: 表示反复回想同一问题, 但未必目的明确或全神贯注(rapt入神的,a rapt audience听得入神的听众)。(另一意思:反刍)。


记忆方法: 1) 首字母记忆： PM MR首相先生(MR. Prime Minister)经常沉思

     
       2)  meditate和thought互释,都表示深入思考，见2017-12-15 想法idea-concept。


       3） 除了第一个ponder直接接宾语，后三个介词都用on。


      4) 按专注程度： meditate>ponder>ruminate>muse

       
      5）沉思的意思是专注仔细的思考mean to consider or examine attentively or deliberately.