Subjective Idealism
Subjective Idealism, also known as Immaterialism, is a monistic metaphysical doctrine which asserts that only minds and mental contents exist. It fundamentally denies the existence of material substance as an entity independent of the mind. The most influential proponent of this school of thought was the 18th-century Irish philosopher George Berkeley, who sought to combat the rising Materialism of his era.
The core tenet of George Berkeley's philosophy is encapsulated in the Latin maxim esse est percipi (to be is to be perceived). Berkeley argued that the objects we perceive—such as trees, stones, and books—are nothing more than collections of 'ideas' or sensations within the mind. He challenged the distinction made by John Locke between primary qualities (like extension and motion) and secondary qualities (like color and taste), arguing that both are equally mind-dependent and subjective.
In the field of Epistemology, Subjective Idealism addresses the gap between perception and reality. While Realism posits a world existing outside of thought, the subjective idealist maintains that we can only ever know our own perceptions. To explain the continuity and stability of the world when no human is observing it, Berkeley invoked a supreme perceiver, God, whose constant observation maintains the existence of all things.
This perspective significantly influenced later developments in Philosophy, particularly the Transcendental Idealism of Immanuel Kant and the Phenomenalism of the 20th century. Critics of the theory often point to the risk of Solipsism, the view that only one's own mind is sure to exist, though Berkeley himself vigorously rejected this conclusion.
For detailed academic analysis, see the Stanford Encyclopedia of Philosophy on George Berkeley and the Britannica entry on Subjective Idealism.