Noumenon, Reason, Happiness

According to Schopenhauer, the noumenal realm consists in undifferentiated 'Will', with respect to which Reason is demoted to operating on phenomena, at the service of empirical, differentiated, individuals, just as Hume argues.  Now, insofar as the principle of Pure Practical Reason  cannot be derived from empirical sources, and is self-evidently efficacious, as Kant argues, it is immune to Schopenhauer's proposed subordination of Reason.  However, once he starts attributing Plurality to the presumably inscrutable noumenal realm, i. e. when he speaks of a multiplicity of Rational subjects, he becomes vulnerable to Schopenhauer's challenge.  But, the perhaps fatal weakness in his doctrine is when he classifies Happiness as a Rational Good.  For, even with the latter as conditioned by Virtue, it leaves the principle wide open to the charge that it is merely part of a circuitous means to an individual's securing of personal Happiness, just as Hume might have suspected.