Modern subjectivism acquires new-found significance in the works of Schopenhauer and later that of Nietzsche. Schopenhauer integrates a number of Eastern concepts (Buddhist striving/suffering, subject-object core) with the Western post-Kantian framework (noumena/phenomenon, thing-in-itself). His first major thought uncovers an important assumption behind Leibniz’s principle of sufficient reason (knowledge/truth must have sufficient explanations); particulars that require explanations presuppose a subject that seek the explanation (subject-object divide is the root of sufficient reasons).  Moreover, sufficient explanations can be differentiated by the categories of the objects referenced; material things have casual relations, abstractions by logic, Mathematical/geometrical constructions by number/space, psychology/motivation by moral reasoning. Explanations, by such categories of objects, shall also be mutually exclusive or unmixed. This argument is used against Kant’s thing-in-itself (a mind-independent object that is inaccessible to human experience) as having caused mind-dependent sensory experience (causality between these objects is a categorical error); sensory experiences are denied their external causes and instead must be related in another way.

Schopenhauer’s answer is a Pantheist one where sensory information are not caused but instead are representations of the Will (two-sides of the same coin) e.g. the appearance of lightning is the representation of the Will of electric potential, the movement of our physical bodies as manifestation of our own wills. The unity between Will and representation in our bodies (subject with object) assigns it greater importance as it is treated as the point of reference to all other objects; acts of the will are instantly objectified by the body and thus one and the same . Will is objectified through a two-tiered approached (Platonic Ideas which are outside space-time, and then by their particulars which are constituted in space-time). The world of appearances is thus a reflection of Will (Panpsychism).

Schopenhauer’s Will is unlike that of the rational/logical self-consciousness of the German idealists of his time; Will is a wholly mindless, aimless, and non-rational urge responsible for our instinctual drives. It is blind-impulse with neither aim nor determination; such a world strives for nothing in particular but only to further fragment (differentiate, individuate) itself (through principles of sufficient reason, law of free-energy will agree). The experience of such fragmentation is human suffering and  frustration. Three solutions are posited:

1. The aesthetic perception or merging with less-differentiated minds (discernment through objects, losing oneself in an object) and access its Platonic Idea; high-art gives spatial-temporal forms to these Ideas which allow others to more closely access them. Music has special importance as they contain the structures of the world itself along a feeling axis.
2. Having compassion for the suffering of others as they are cut from the same cloth (from the act of Humanity itself). Recognizing the universal consciousness in everyone curbs the energy of blind impulse. Determining our own innate propensities/characteristics (self-knowledge) leads to tranquility.
3. Asceticism or minimizing one’s desires reduces suffering and frustration; it is a form of regression into a less differentiated state and the curtailing of will to knowledge.