Basically, Berkeley writes that the only things that are real or existing are either (1) the active, perceiving mind or soul, or (2) the passive perceived things. He argues that we can only have direct knowledge of the sense-data, but this in no way implies the existence of some real, unperceived, independent, objective thing. Thus the only safe thing to assume is that the only thing that is real is the mind and the things it perceives (the sense-data).
I can definitely see his arguments and their validity, he certainly has a rigid system with a strong defence. One thing that comes to mind: Berkeley claims that we (1) perceive ordinary things (cars, knights, the sun), (2) We only perceive ideas (I take this to be the sense data or the sense impressions, the objects of thought). From this he infers that all ordinary objects ARE ideas. In other words, there is nothing besides what the mind perceives. But isn't there any reason to suppose that all the mind perceives must come FROM SOMETHING? Descartes frantically attempted to provide knowledge of the extra-mental from a bastion of defence in the cogito argument. But his support is the existence of a benevolent God, a weak argument in my eyes. Locke argues for a distinction between the primary qualities of things, and the secondary qualities of things. He says the primary qualities are the thing-in-itself (extension, motion, etc) and the secondary qualities are entirely subjective and constructs of the mind. But Berkeley seems to demolish this argument in his first dialogue. To make matters worse, even Immanuel Kant seems to admit that we can't REALLY have knowledge of the Noumena (the thing-in-itself), and would thus be a good ally of Berkeley.
Is there any point one could attack Berkeleys view? How would his predecessors respond to his attacks?