The split posited by Kant between phenomena and noumena, effects and causes, creates a strange kind of reductio ad absurdum around subjects and subjectivity. The subject, in Kant, owing to his/her phenomenal appearance in a world of things, is (exists/subsists) as phenomena and noumena, effect and cause- yet, self-reflection, originating from conceptions of the understanding (employing the Kantian cognitive model) reveals the subject (bound to the unity of apperception or cogito) to evince the ability to objectify his/her own consciousness as an object-in-itself, as phenomena and effect- yet this consciousness, inaccessible to others, constitutes the subject's substance/causality for all perceiving subjects other than itself; in other words, the noumena, the thing-in-itself, is also an object for the perceiving subject in whom it inheres- creating the aforementioned reductio ad absurdum when the dialectic, for logic, is around self-reflection and potentialities of the subject's self-knowledge. It leads to the ineluctable reflection, for the perceiving subject- is the noumena there at all; and, if it is, is it completely inaccessible even for the perceiving subject, owing to the constraints of sensibility and understanding?
In another fashion: is the noumena, the thing-in-itself (substance, causality) so merely a subsistent, rather than existent, entity, that what is behind phenomena/effects are a reductio ad absurdum of more effects, more phenomena, and substance impossible even to hypothesize; and, if what constitutes the noumena is subsistent matter (half perceptible/half imperceptible), why the noumena should be presupposed as anything (in our dialectics) but this posited chain of effects/phenomena, in terms of what is available to our understanding; and, if this is so, why Kant's distinction can never touch the subject in such a way that, through self-reflection, the noumena can be grasped or encompassed. In other words: the subject cannot necessarily merely be the noumena for other subjects, or for itself. As to whether subjects should project likeness onto other subjects- the sameness in human subjectivity, as judged by the phenomena of consistent human behavior, may be determinative of the dialectical necessity of a quantitative judgment on this level- of half-causes, half-noumena, half-substance; if Kant's conceptions are to address the empirical reality of human subjectivity to the fullest possible extent.