Your proposition, though, is about gravity, not a proposition itself.
The problem isn't with your evidence, which is fine, it's with your proposition, which is essentially a restating of the evidence. It cannot be doubted, because of its tautology --it's automatically true. It requires no disbelief. In order to compose a decent proposition for this exercise, it has to be something for which belief in it can be doubted. It has to be disbelievable. That is the proposition that needs justifying.
I was thinking of a scenario where a person comes up with a hypothesis about how the world works, then tests it and confirms that it's true by observation.
I admit that "stuff falls down when you drop it" is a very simplistic hypothesis to test, and it would be hard for a person to function in the world without having realized that this happens, but I was assuming that the person believes but does not know that this occurs prior to testing the belief.
If you want something more substantial, how about this:
- I believe that when you drop something, it falls down
at a constant acceleration rate if not for aerodynamic drag.
- I test this by rigging up a vacuum chamber (thereby eliminating drag) and dropping various objects in it in a systematic way.
- I observe that all the objects fall at the same acceleration rate.
At this point, I would be justified in believing that my hypothesis is true. Since I believe it and it is actually true, I would then
know it.