Why did Immanuel Kant criticize Berkeley’s idealism?

Immanuel Kant criticized George Berkeley’s idealism—which he termed "dogmatic idealism"—primarily because he viewed it as inconsistent with the reality of an external world.12

Kant argued that Berkeley’s position effectively denied the independent reality of space. Berkeley maintained that properties such as extension and impenetrability exist only within the mind, leading to the conclusion that material objects are merely collections of ideas.312

In his Critique of Pure Reason, specifically under the "Refutation of Idealism," Kant sought to prove the existence of "things outside us". He countered Berkeley by proposing that space and time are not properties of "things-in-themselves" but are instead forms of "sensible intuition"—the mode through which the mind is affected by objects. By combining transcendental idealism (the view that we experience only appearances) with empirical realism (the view that objects exist external to the mind), Kant argued that being in space and time is a necessary condition for an object to be experienced, thereby guaranteeing the reality of external objects without requiring an indefensible inference to hidden causes.1243