I've been giving this some thought as well, mainly because it has to do partly with the philosophy of religion. I'm leaning towards the idea that, depending on the nuances of one's definition of the word, we can't actually know if something is "objective", simply because it's meaning seems to prevent that. For example, consider proposition "X". Let's imagine that proposition X is objective; meaning, proposition X is the case regardless of whether anyone actually thinks it is the case or not (for the purposes of this thought experiment, "everyone" is interpreted to mean "all humans"). The two most obvious ways for us to be able to know if proposition X is objective, is if alien's, not being human, hold an opinion on proposition X, or if gods, not being human, hold an opinion on proposition X. Outside of those two exceptions, things could still be objective, but there doesn't seem to be a way of knowing what those things are, as you mentioned. - starpilot149