Because a rock is still a rock regardless of if humans are around to observe it.
Nice, the old "argumentum ad lapidem".
I understand we find it easy, in fact, it's intuitive and downright "obvious" that "things" "exist" "independent of observation and evidence".
But what you seem to be missing is that this "common sense" (a priori "fact") is a de facto philosophy in-and-of-itself, namely Naive Objectivism (naive realism).
Any "thing" we don't have evidence of is an aspect of NOUMENON.
Part of the NOUMENON is potentially "knowable", Mysterium Invisus and part of it is fundamentally unknowable, Magnum Mysterium.
I'd say that "existence" is probably not the best word to describe noumenon (mainly because the definition of "exists" requires empirical verifiability). I believe it is a mistake to imagine noumenon as some sort of "thing" when it is merely an amorphous concept that acts as a place-holder for both "what we don't currently know" (Mysterium Invisus) and "what may be fundamentally unknowable" (Magnum Mysterium). For example, noumenon might be eleventy-trillion layers of sci-fi multiverse, noumenon might be an elaborate alien computer simulation, noumenon might be Brahma's dream, noumenon might be a single super-intelligent (but not omniscient) demiurge that we humans are merely appendages of. In all likelihood, it is conceptually, literally, ultimately and completely beyond our ability to comprehend. All of this makes it very very very difficult for me to believe that we can consider (with any degree of confidence whatsoever) that noumenon is itself comprised of 100% pure, uncut, "objective reality". I mean since noumenon may involve a great many (likely) possibly subjective layers (simulation/dream/multiverse) below our primitive perceptions, although we can deduce with the confidence afforded us by our logic, that there must be, at some level, "real" and "true" and "objective" "reality", we cannot have any confidence that what we are able to perceive has anything-at-all to do with the-hypothetical-objective-essence directly. It's like the old story of the princess and the pea. Clearly there is "something" under the bed, but what are the chances that a normal person would be able to detect it through ninety-nine high-quality mattresses(?).
I gave a tautology, which is a necessarily true statement, rather than making the fallacy of a stone. Just because I used a stone as example doesn't mean I gave a fallacy involving a stone. The fact that I asserted that a rock is a rock doesn't mean that all that I was doing is making a baseless asseriton. Far from baseless, "a rock is a rock" is necessarily true on account of it being tautologically true. "A rock is a rock" is on a par with A=A which is the law of identity which is the fundamental law of logic and cannot be false.
Nope. You failed to make your definitions explicit.
And because you failed to make your definitions explicit, you've inadvertently made an "appeal to common sense", which is a naked "appeal to ignorance".
A noumenon has to be a thing for it to be anything at all, as far as I'm concerned. Either there is a such thing as X or there isn't. And "There's no such thing as X" equates to "X doesn't exist". Either the noumenon is a thing or it is nothing at all.
I like where you're going with this.
NOUMENON is a special case.
In order for a "thing" to properly "exist" we must have some way of detecting it empirically, either directly or indirectly.
NOUMENON is not detectable empirically, but it is LOGICALLY NECESSARY.
We only "know" of NOUMENON, because we can deduce that we DO NOT currently know "everything".
Therefore, NOUMENON "exists" more like a category, and less like an actual "thing" (even though it may contain currently undetected things).
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Your scathing critique is requested.