Paul Guyer's book "Kant" (Routledge).

He says, "To be able to decipher the nature of a thing does not imply that a person is directly acquainted with all aspect of this thing"... Kant "reveals to us the nature of the noumenon. But this is not to say—and Kant himself acknowledges this—that he can speculate as to what the noumenon is (if
anything at all) besides from manifesting or objectifying itself as the human will.

I reconstruct the argument as:

(i.) If some object/thing A manifests as some object/thing B, then B is the nature of A,

(ii.) The noumenal world manifests as the human will; thus,

(iii.) the human will is the nature of the noumenal world.

What is the name of the form...modus tollens I think? It seems deductively valid, but unsound because of (i.). e.g. i can say the “if depression manifests as body-trembling, then a body-trembling is the nature of depression”. But this seems to be false since body-trembling is not the "nature" of depression, but maybe just one symptom...
Any feedback much appreciated!