But in fact, logically speaking at least, variability, although it's a requirement for unpredictability, doesn't guarantee unpredictability.

但实际上，至少在逻辑上讲，可变性虽然是不可预见性的必要条件，但是不可预见性却不是必然。

耶鲁公开课 - 死亡课程节选

which was so nice and so unnecessary, but she's wonderful.

真的很好，虽然没必要，但她真是太好了。

就业的方法 - SpeakingMax英语口语达人

It's not necessary to do that, but it can be quite convenient.

虽然没有必要这样做，但是这样做是很方便的。

麻省理工公开课 - 热力学与动力学课程节选

Why? Because good will and idealism, while necessary,they are not sufficient.

为什么呢？因为美好的意愿和理想主义，虽然必要，但却是不够的。

哈佛公开课 - 幸福课课程节选