|No your reasoning is not sound.|
If one asserts X exists, then reasonably someone else could ask for evidence or be skeptical. But skepticism is not the same as belief in X's non-existence. If one wants to positively assert the absence of X, then they would have as much burden of proof as someone arguing for X. You aren't just saying "I don't know about the idea natural rights, no one has shown me any proof", your saying "natural rights don't exist". For things issues where there isn't any reason to expect that either presence or absence would lead evidence, only the agnostic position can reasonably impose serious burdens of proof on those that disagree.
Plato pointed out that we have an innate idea of equality by pointing out that even though there aren't two sticks that are perfectly equal we can imagine that they exist......That's pretty good evidence for an innate concept of equality.
Its argument not evidence. Most (and probably for very strict definitions of proof, and non-expansive definitions of philosophy, all) philisophical ideas only get backed up by argument, not by any sort of concrete evidence. That's also the case of a number of non-philosophical points, about which evidence is limited.
"For example - No one can provide any actual evidence of civilizations, (or even the existence of life) in the Andromeda galaxy." Its an almost perfect analogy. We have no evidence, but we have reason to believe there is life, and probably even civilizations there. That issue, like the idea of natural rights, is an area where we have no actual evidence, or current or reasonably foreseeable ability to get evidence. You can (and have) postulated a theoretical way evidence could be obtained, but that way is impossible at this time, and may never be possible. And even should it become possible, the salient point here is that at this time we have no evidence.
That's a bad analogy. We haven't looked for life there. Had we gone to the Andromeda galaxy and visited all of the likely planets and found no life, that would be evidence for a lack of life there.
If I said that there is an invisible unicorn in my garage you'd assume that there is one?Invisible unicorns would be a physical thing. We have extensive evidence that animals tend not to be invisible and more generally tend to leave evidence. We have knowledge understanding that if the only change made to an animal would be to make it invisible, that it would still leave evidence (it would eat, eliminate waste, make noise, move other objects, leave footprints, etc.) If your arguing that there is an invisible unicorn in your garage, and I actually cared to make the effort to check (and knew where you lived, and was invited to check etc.) I would ask you to present the evidence that we have good reason to expect (unlike with philosophical concepts, or life in another galaxy given our current knowledge and ability, both of which would be cases where we have no reason to expect evidence). If you could present it, and it was solid, and other people noticed it, and tests where done to show it wasn't fake, then at some point I probably would believe that an invisible unicorn existed in your garage.
But the standard of evidence would be high, because such a thing (unlike either natural rights or life in other galaxies) goes against all human experience.
If you couldn't present any evidence, but argued that its an invisible unicorn that doesn't make a sound because its inaudible, and doesn't leave evidence of having eaten food or eliminating waste because it doesn't need to eat, and doesn't knock things over or get noticed when we feel around for it, because it is small, quick, smart, shy, and careful, and doesn't leave foot prints (even if say you drop paint or powder all over the floor) because it floats above the ground (and does so without needing to direct any powerful air currents downwards) etc. then you are making a far more extreme claim, that goes even further against all human experience and to a great degree against current scientific knowledge, so the burden of proof on you would be even higher, even though it would be harder to get. But still you could reasonable say that I had not proven that there is no invisible unicorn in your garage I would merely have shown that it being there would imply that much of human experience and scientific knowledge is false, at least in the rare corner case of your unicorn. Which would be a powerful argument against the idea of a unicorn in your garage, but would not be proof.
But neither human rights, nor extra-galactic life or civilization, contradicts human experience or scientific knowledge. If we are to demand evidence to have any opinion the only opinion reasonably left is to be agnostic on both issues (and many others). If you don't think that evidence need to exist in order to have opinions, when the question is one where evidence would be unlikely to exist, then you reasonably can have either opinion.