Why would he say that? Because no one can perfectly interpret the quantum mechanics? Or something else?
更新1:
要是 [understand ] 泛指一切能被直觀所理解的物理現象,那麼相對論也描述宏觀和高速狀態下的物理現象,但從來沒有人說他們不懂special relativity and general relativity. 但我看每一本科普著作,作者都以費曼這句名言表示他們對量子力學的不理解。連教授也跟我們說,everyone can do QM, but no one understand QM. 同學問他,那你呢?他說I don't understand.