What is George Berkeley's master argument about imagined objects?

George Berkeley’s "master argument" addresses the error he believed people make when they attempt to conceive of objects existing independently of a mind. He argued that when a person imagines objects, such as trees or books, existing "and no body by to perceive them," they are failing to account for the entire situation.1

Specifically, Berkeley contended that the person imagining these objects is "omitting" the perceiver from their mental model. He maintained that imagined objects are necessarily imagined as being perceivable, meaning the situation remains a two-term relation between a perceiver and the perceived. By failing to recognize that the act of imagining requires a mind to do the imagining, one mistakenly assumes the objects can exist outside of any perception. For Berkeley, because all sensible objects are things we perceive by sense, and nothing is perceived besides our own ideas, it follows that all such objects are ideas that exist only within a mind.1