這裏的Things seen are mightier than things heard其實寫全了就是：
Things that are seen are mightier than things that are heard.
seen和heard這裏作為形容詞,表示“被聽到的、被看見的”,這壹類由分詞而來的形容詞是要後置的
這裏的seen和heard是壹個詞的,短語形式的例如：
The story believed by all the children is actually not true.
所有孩子都相信的這個故事其實不是真的.
believed by all the children就是壹個後置的分詞短語,修飾story