In philosophy, subjective idealism is a doctrine which holds that the material world has no significant existence or value aside from that given it by the mind. The doctrine holds that one’s ability to perceive an external entity is due to the mind’s cognitive capacities rather than to any objective properties of the entity itself. Knowledge, in turn, derives from the ideas that emerge from such perceptions. Subjective idealism is closely linked with the doctrine of immaterialism, which denies the existence of matter and holds that only spiritual beings and incorporeal forces truly exist in a metaphysical sense.
One of the most important pioneers of subjective idealism was the Anglo-Irish clergyman and philosopher, George Berkeley (1685–1753) (for whom, incidentally, the city that hosts the University of California was named). Berkeley famously asserted that esse est percipi vel percipere—to be is either to be perceived or to perceive. In response to the growing influence Newtonian science and more mechanistic perspectives during his time, Berkeley intended his doctrine as an assertion of the reality of the spiritual world, and as a defense of God’s existence. He reasoned that, since the existence of things cannot entirely depend on their being perceived intermittently by different people, the existence of the material world must ultimately be due to its being perceived, and revealed to humans, by the mind of God. Still, other philosophers have challenged whether Berkeley’s rationale truly proves God’s existence.