英语翻译The bridge by which we cross from tragedy to comedy and

英语翻译
The bridge by which we cross from tragedy to comedy and back again is precarious and narrow.We find ourselves in one or the other by the turn of a thought; a turn such as we make when we turn from speaking to listening.神马意思?观点是什么?
没有全文啊，老师让我们以这句话作为演讲的内容。可我不知道它在说神马。是说思想变化快啥的么？