Simplify the following text for a Grade 11 native English-speaking person.
Original: George Berkeley ( ; 12 March 1685 - 14 January 1753 ) , also known as Bishop Berkeley ( Bishop of Cloyne ) , was an Anglo-Irish philosopher whose primary achievement was the advancement of a theory he called `` immaterialism '' ( later referred to as `` subjective idealism '' by others ) .
Simplified: George Berkeley ( 12 March 1685 â `` 14 January 1753 ) , also known as Bishop Berkeley , was a philosopher . His main philosophical achievement was the advancement of a theory he called `` immaterialism '' ( later referred to as `` subjective idealism '' by others ) .