据我们所知，有“已知的已知”；有些事，我们知道我们知道。我们也知道，有“已知的未知”，也就是说，有些事，我们现在知道我们不知道。但是，同样存在“不知的不知”；有些事，我们不知道我们不知道。——DonaldRumsfeld
桑迪·里扎,于里·莱瑟森,肖恩·欧文,乔希·威尔斯. Spark高级数据分析（第2版） (Chinese Edition) (Kindle位置2468). Kindle 版本.
Reports that say that something hasn’t happened are always interesting to me ， because as we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns: the ones we don’t know we don’t know."
——DonaldRumsfeld
