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


meditate: implies a definite focusing of one's thoughts on something so as to understand it deeply: the sight of ruins prompted her to ~ upon human vanity.            vanity虚荣     (meditate和thought互释，都表示深入思考，thought在想法idea-concept组)    冥思中的冥表示“深沉深奥”，如冥思冥想            


muse: suggests a more or less focused and persistent but languid and inconclusive turning over in the mind as if in a dream, a fancy, or a remembrance: ~d upon the adventures of the heroines of gothic novels.       缪斯Muse是古希腊和罗马神话中执掌诗歌、音乐和其他文学艺术分支的九位女神之一，还比喻创作动力和灵感       (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 suggests little of either purposive thinking or rapt absorption: the product of fifty years of ~ing on the meaning of life.      (rapt入神的, 如a rapt audience听得入神的听众)    (ruminate另一意思:反刍)


ponder仔细思考: 仔细衡量和思考问题，meditate沉思冥想: 专注思考以深入理解，muse若有所思: 慢慢的想(turn over in the mind翻腾反复)如梦幻想象回忆，ruminate反复想: 表示反复想同一个问题，不一定认真仔细


记忆方法: 1) 首字母PM MR首相先生(MR Prime Minister)<==经常沉思          这组词除了第一个ponder直接接宾语，后三个都用介词on/upon
         
               2) 沉思的意思是专注仔细的思考mean to consider or examine attentively or deliberately.首字母PM MR首相先生(MR Prime Minister)<==经常沉思