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 George Berkeley was an 18th-century Irish philosopher and bishop best known for his theory of immaterialism (also called idealism).

Core idea

His most famous claim is:

“To be is to be perceived” (esse est percipi)

What he meant

Berkeley argued that:

• Physical objects do not exist independently in the way we usually think.
• Instead, objects are collections of perceptions (what we see, feel, hear).
• So a “tree” exists because it is being perceived—by you, or ultimately by God.

In his view:

• You never experience “matter itself,” only sensory ideas.
• The mind is what experiences reality.
• God continuously perceives everything, which keeps the world consistent even when no human is observing it.

Simple example

If a tree falls in a forest:

• Common view: it still exists as matter whether or not anyone sees it.
• Berkeley’s view: it exists because it is always perceived (at least by God), so it doesn’t depend on human observation alone.

Why he matters

Berkeley is important because he:

• Challenged the idea of a mind-independent material world
• Influenced later debates in philosophy about perception and reality
• Helped shape discussions that eventually connect (loosely) to modern ideas about observation in physics

George Berkeley

If you want, I can also  explain how his ideas differ from Descartes or how they connect (and don’t connect) to quantum “observer effects.”