Let's call someting invisible not only it can't be seen, not only it can't be observed, but it can't be detected at all.

让我们把一些不仅是看不见的,不仅是观察不到的，而且是完全检测不到的，东西称为无形的。

耶鲁公开课 - 死亡课程节选

Two, different possibility, can't be observed.

第二个不同的解释，观察不到的。

耶鲁公开课 - 死亡课程节选