Why did George Berkeley reject John Locke's idea of an imperceptible substance beneath an object's qualities?

George Berkeley rejected John Locke’s concept of an imperceptible "substrate" or substance underlying an object's qualities primarily because he believed it was logically incoherent and unsupported by experience.12

Berkeley’s rejection was grounded in his empiricist commitment to the idea that all human knowledge is derived from perception. His arguments against Locke’s substrate included:2

• Inaccessibility: By definition, Locke’s substrate was described as imperceptible—something "behind" or "underlying" ideas that could not be directly observed. Berkeley argued that if all knowledge comes from sense experience, it is impossible to have any clear idea of something that is, by nature, unobservable.12
• Redundancy: Berkeley contended that there is nothing more to an object than the collection of its sensible qualities. He argued that physical objects are simply groups of ideas; therefore, positing an additional, hidden substance to hold these qualities together is unnecessary and adds nothing to our understanding.123
• Contradiction: Berkeley maintained that because qualities like extension and motion are themselves ideas, they must be perceived. He rejected the notion of "matter" as an unsensed, underlying entity, asserting that such a concept is a contradiction in terms.1