However, it has never been directly observed for a black hole.

Evolution has been directly observed, do try to pay attention.

Rather, it is a mental state that cannot be directly observed.

I imagine that would still make them rather difficult, if not impossible like L3, to directly observe.

Students should also directly observe the difference between sand and your "gold."

They could not directly observe each other without electronic assistance.

Patterns may be directly observed with any of our five senses.

In any case a diaphragm itself or its position could not be directly observed.

And, as such, the velocity of those waves has never been directly observed.

However, a significant amount of this starlight is not observed directly.