6 ideas
Everything that exists for knowledge exists only as object in relation to a perceiving subject, so the world as experienced is representation. Its structure of space, time and causality comes from the forms of the knowing subject's cognition, not from things in themselves.
Each of us knows our own body in two ways: from outside, as an object among objects, and from inside, as direct willing. An act of will and the body's movement are one event given in two ways. By analogy, the inner essence behind all appearances in nature is will.
The will is not rational choice. It is a groundless, endless urge that shows itself in gravity, plant growth, animal instinct and human desire alike. Because it has no final goal, every satisfaction only clears the ground for new craving.
Desire comes from lack and is therefore painful, and satisfying it brings no positive state, only the brief removal of that pain. Once a need is met, boredom follows, so suffering is built into existence and happiness is merely negative.
In aesthetic contemplation the individual becomes a pure, will-less subject of knowing who sees the Platonic Idea of an object rather than its usefulness. This briefly frees a person from desire. Music ranks highest among the arts because it copies the will itself, not its Ideas.
The separation between individuals is an illusion produced by space and time, the 'veil of Maya.' Seeing through it grounds compassion, because another person's suffering is recognized as one's own essence suffering. Carried to its end, this insight leads to ascetic denial of the will, the saint's quieting of all desire.