[Leaning Right]

but isn't sight passive???

Yes. The effects are there even if a human is not observing the situation.

Suppose that a free electron is sitting on a lab table. And the overhead lights are on. Particles of light will "bounce" off the electron and cause the electron to shift around. It's like how throwing balls at a cardboard box will cause the box to shift around.

So a human does not cause the uncertainty in where the electron really is. The human just notices the uncertainty.

[Chode]

now THAT makes sense...