双重否定 (Double Negation)

　　两个否定连用，彼此相互抵消，表示肯定的意义，称为双重否定，如：
　　　I can't not obey. (= I have to obey.)

　　双重否定通常有两种用途：
　　1. 加强语气，这种双重否定表示的肯定有时比肯定式表示的肯定语气更强，如：
　　　No one has nothing to offer to society. (= Everyone has something to offer to society.)
　　　He was never dissatisfied with my work. (= He was always satisfied with my work.)

　　有些双重否定很难甚至无法换用肯定形式来表示，如生硬地改为肯定形式，将大大影响原意，如：
　　　No investigation, no right to speak.（没有调查研究，就没有发言权。）
　　　You can't make something out of nothing.（巧妇难为无米之炊。）

　　2. 使语气委婉，如：
　　　I have brought back what you want not without risk and danger.
　　　Nothing is impossible if you try your best.