Why did Arthur Schopenhauer consider the Upanishads, Plato, and Immanuel Kant central to his philosophical system?

Arthur Schopenhauer considered the Upanishads, Plato, and Immanuel Kant to be the foundational pillars of his philosophical system.1

• Immanuel Kant: Schopenhauer’s work began with the Kantian distinction between phenomena (the world as appearance) and noumena (the thing-in-itself). He adopted Kant’s view that human cognition is structured by space, time, and causality, meaning we can only perceive the world as a representation rather than as it truly exists.21
• Plato: Schopenhauer integrated Platonic thought into his own framework, particularly regarding the nature of reality and the role of aesthetics in providing metaphysical insight.1
• The Upanishads: Introduced to these ancient Indian scriptures by the Orientalist Friedrich Majer, Schopenhauer found in them a profound resonance with his own conclusions. He viewed the Upanishadic teachings as essential to his understanding of the world, helping to shape his unique synthesis of Western idealism and Eastern mysticism.1

Together, these influences supported his development of the "philosophy of will," which posits that the underlying essence of all reality is an insatiable, nonrational life force.21