Berkeley's theory, which sounds like something out of The Matrix, argues that physical things do not really exist except as ideas, until they are perceived by the senses of people perceiving those physical things. His expression “to be is to be perceived” summed it up. Hence as you sit at your computer at your desk at home, you see the images on the screen, you feel the keyboard keys, you hear the sound of the keys and the sounds from the speakers. If you are at work, how do you know that the computer on your desk at home still exists? It exists in your mind and memory, but that does not equate to existence in fact. You believe it still exists but you don't know that it does. To validate existence of material objects we need to use our senses.

Okay, I can dig that, I hear you say, the computer may have been destroyed in a fire and no longer exist. But Berkeley takes it further. He is not saying that existence is dependent on existence, he says that when not perceived, an object ceases existence.