Now this is a side issue. There's your incorrect assumption that you have no obligation to demonstrate negative assertions. That's just silly. Especially given the agnostic option.
But then you also differ from Kant who clearly considers it not only possible that noumena exist, and not just phenomena, but even considers the existence necessary for things as important to Kant as morals. Some of his statements are extremely strong about the absurdity of thinking there are no noumena. In any case, Kant certainly is not asserting they do not exist.
General ontological position on Noumena
“Although we cannot know these objects as things in themselves, we must yet be in a position to think them as things in themselves; otherwise we should be landed in the absurd conclusion that there can be appearance without anything that appears.”
— Critique of Pure Reason, Bxxvi
“It is indeed necessary to assume behind appearances something else which is not appearance, namely, things in themselves, though we cannot know them at all as they are in themselves.”
— Critique of Pure Reason, A250
“But we cannot possibly assume that the sensible world is the only possible mode of intuiting all things, in which case it would follow that nothing would remain over for the noumenon.”
— Critique of Pure Reason, A249
“There must indeed be some ground, independent of the conditions of sensibility, which provides for the possibility of sensible phenomena, and that ground we may call the transcendental object, or simply noumenon.”
— Critique of Pure Reason, A251
“Things in themselves, which lie beyond the field of sensibility, are indeed real; yet they cannot be known by means of the senses or by the categories.”
— Critique of Pure Reason, A30/B45
“Now we find that our thought of the relation of all knowledge to its object carries with it something of necessity, since objects are not given to us as things in themselves but only as phenomena. This cannot be without there being something distinct from sensibility to correspond to them, that is, as a thing in itself.”
— Prolegomena to Any Future Metaphysics
“Thus there certainly remains a place open for some kind of other knowledge, namely knowledge of things as they are in themselves… yet without pretending to claim this knowledge.”
— Critique of Pure Reason, A277/B333
Specific Noumena necessary for us being moral agents - and Kant obviously thought it was possible for us to be moral agents or why bother spending so much time on explaining what is moral
“The summum bonum, which is the necessary object of a will determinable by the moral law, is possible only on the supposition of the immortality of the soul, so that a progression toward that perfect conformity of dispositions to the moral law is prolonged infinitely.”
— Critique of Practical Reason, Book II, Chapter 2, Section 5, also referenced as (5:122-123) in the Akademie edition.
“This endless progress is… possible only on the supposition of an infinitely enduring existence and personality of the same rational being (which is called the immortality of the soul).”
— Critique of Practical Reason, Book II, Chapter 2, Section 5 (5:122-123 in the Akademie edition)
“It is morally necessary to assume the existence of God…as the supreme cause of the highest good.”
— Critique of Practical Reason, Book II, Chapter 2, Section 5, referenced as (5:125) in the Akademie edition.
“Freedom must be presupposed as a property of the will of all rational beings. It is not enough to ascribe freedom to our own will on whatever grounds…; we must necessarily attribute it also to all beings endowed with reason and will.”
— Groundwork of the Metaphysics of Morals, Part III, Section 1 (4:455 in the Akademie edition)
“For a rational being, who is conscious of his causality with respect to certain effects in the world, reason must regard himself as free. Otherwise, he could not think of himself as the author of his actions.”
— Critique of Practical Reason
“We must be able to hope for an existence that can be endlessly prolonged, if we are to approach the perfect fulfillment of the moral law.”
— Critique of Practical Reason
“The three propositions of the pure practical reason…are: freedom, immortality, and the existence of God. These are not theoretical dogmas but postulates of pure practical reason.”
— Critique of Practical Reason
“It is only as a moral being that man is capable of having a ‘good’ will, and for this to be possible…freedom, the existence of God, and immortality are indispensable as ideas of pure practical reason.”
— Critique of Practical Reason
“Since the moral law is a law of the causality of free agents, and thus of the noumenal world, it binds all rational beings as such.”
— Critique of Practical Reason, Book I, Chapter 1, Section 7 (5:28-29 in the Akademie edition)
So, in general why does VA's
I don't have to justify it because it is a negative assertion defense fall apart?
1-Negative Assertions Can Be Reformulated Positively: Most negative claims are just the inverse of positive assertions and can be rephrased as suchFor example, "noumena and things in themselves do not exist" is equivalent to saying, "the only reality is phenomena, or observable experiences." This rephrasing makes it clear that a positive claim about reality is being made, and thus it requires justification. And this only becomes hilariously more clear given that VA's and Kant's epistemology demands that we cannot know what is beyond the phenomena we experience and what those things would be like. Of course Kant asserts that it would be silly to assume there was nothing.
2-Agnosticism Creates a Burden of Proof for Any Definitive Position: If one has the option to remain agnostic, then any assertion—positive or negative—that aims to refute this neutrality requires justification. Without evidence or argumentation, one hasn’t shown why the agnostic position is untenable, and thus both positive and negative assertions should be expected to offer support. To the agnostic they are both very similar types of claims and both require justification.
3-Negative Claims Often Imply a Certain Knowledge or Authority: Asserting the non-existence of something, especially something as conceptually complex as noumena, implies a claim to know the boundaries of reality or experience. To credibly claim something does not exist, one must justify how they know this and address potential counterarguments, especially when the claim is metaphysical.
3-Be consistant in rational discussion: The idea that all claims require justification is fundamental to critical thinking and rational discourse. The habitual by VA fiat decision that negative claims don't require justification is merely silly. If I said: there are no phenomena - I am sure VA would expect justification. If I said he isn't a human, this would require justification and he would expect it. He has told us about courses he has taken. If I said he never took those courses, my certain assertion of this would require justification - if positive claims require justification, there is no reason to make an exception for negative claims, especially given point 1 above.
4-Negative Assertions Can have just as many postential implications and consequences: Negative claims can have just as much impact on belief systems, practical implications, and worldview as positive claims. A claim like "noumena do not exist" isn’t neutral; it actively challenges certain metaphysical perspectives. To influence others’ beliefs or to reshape existing frameworks, even negative claims must be justified.
5-Common Sense and Tradition Add an Extra Layer of Responsibility- in the sense that of course, if you have any expectation of people believing you, and it goes against those categories, well of course, if any has an obligation, you do : When a negative assertion directly contradicts established ideas, traditions, or common intuitions, there is a higher expectation for justification. I mean, in a sense no one is obligated to justify anything. We can run around and assert, given free speech, and no one can force someone to justify, at least not in discussion forums. But to assert that positive claims entail and obligation to be justified but not negative ones, is silly. It also implies that the person has not justification for their negative claims, or why in a discussion forum would they assert something? What assert what you think others should agree with - given all the insults flung at realists - and not justify? And the funny thing is: VA is making a negative claim that goes against Kant. Kant's positions are not common sense, but he did start one philosophical tradition, and yet VA who treats Kant's texts in a Biblical fashion (unless they contradict him), would, one would think, consider himself obligated to justify positions that go against the prophet.
So, first off the category is incoherent (see 1). Then it assume binary options and looks very silly to agnostics who can rightfully expect justification for either claim to knowledge about noumena. (see 2) The idea should be dead after those 2, but the other points show how silly VA's claim is. And then his claim that Kant is like him on this issue. Hardly.
I think he also conflates epistemic issues with ontological ones, where even Kant recognizes and asserts very clearly the difference here. Can VA admit any part of this????
Nah. TMT theory can tell us why.