What are you basing this distinction on?
I would say that object observes an event if it changes its state in response to this event. Yes, that's a very low bar. First, gravity, isn't an event, so "observe" is an awkward word. We can instead "measure" and then observe the results. Of course, if the gravity did change, the rock would presumably change it's shape a tiny bit, which we may or may not count - that's fine, "observation" is supposed to be on a spectrum.
Using Eliezer's formulation of "making beliefs pay rents in anticipated experiences" may make the relevant point clearer here. Specifically, what's an "experience"?
Experiences are brain states, beliefs are also stored in the brain. Eliezer's advice is equally good both for you and for a roomba, regardless of which of you is supposedly conscious. It may not work for plants or ice though - I don't think I can find anything resembling beliefs in them, and even if I could, there would be no process to update them.
(This post grew out of an old conversation with Wei Dai.)
Imagine a person sitting in a room, communicating with the outside world through a terminal. Further imagine that the person knows some secret fact (e.g. that the Moon landings were a hoax), but is absolutely committed to never revealing their knowledge of it in any way.
Can you, by observing the input-output behavior of the system, distinguish it from a person who doesn't know the secret, or knows some other secret instead?
Clearly the only reasonable answer is "no, not in general".
Now imagine a person in the same situation, claiming to possess some mental skill that's hard for you to verify (e.g. visualizing four-dimensional objects in their mind's eye). Can you, by observing the input-output behavior, distinguish it from someone who is lying about having the skill, but has a good grasp of four-dimensional math otherwise?
Again, clearly, the only reasonable answer is "not in general".
Now imagine a sealed box that behaves exactly like a human, dutifully saying things like "I'm conscious", "I experience red" and so on. Moreover, you know from trustworthy sources that the box was built by scanning a human brain, and then optimizing the resulting program to use less CPU and memory (preserving the same input-output behavior). Would you be willing to trust that the box is in fact conscious, and has the same internal experiences as the human brain it was created from?
A philosopher believing in computationalism would emphatically say yes. But considering the examples above, I would say I'm not sure! Not at all!