He follows Kant in distinguishing between conscious phenomena and unknowable noumena that must be conceived as underwriting the coherence that experience finds in the fluctuating phenomenal manifold.  The noumena are “speculative”, by which Herbart does not mean objects of intellectual intuition, but rather the transcendental conditions of the synthetic unity of objects of experience. Yet where Kant held noumena to be unknowable, Herbart takes the further step of ascribing to every existent “an act of self-preservation”, a notion as opaque as it is important to his psychology (SW II: 195).  The idea seems to be that every being must have an inherent force holding it together, or there would be no reason to suppose its continued existence: “the idea of a force to maintain one’s identity” just is “self-preservation” (Beiser 2014: 120).