As a preliminary people might like to read my brief introduction to Berkeley's metaphysic:
A very brief introduction to Immaterialism
Incidentally, what I haven't mentioned before is that I did part of a Ph.D thesis on Berkeley's metaphysic; specifically on immaterialism's implications for the ontological status of the microscopic realm i.e in what sense do objects/processes exist that are too small to be seen with the naked eye. I never completed it, but I think I have an excellent grasp of his ideas.
Alleged Difficulties for Immaterialism
I have just read the following recent article:
Mind over matter: the contradictions of George Berkeley
The author says:
"Some asked how hallucinations fit into his picture, alleging that they sever the link between ideas and reality in some distinct way his system cannot account for. Another classic objection has it that the corollary of believing that things do not exist unless they are being perceived is that objects must be continually popping in and out of existence depending on whether they are being looked at or not, which is metaphysically untidy to say the least."
First of all, it's worth pointing out that the way of establishing whether something is a hallucination or not is precisely the same regardless of whether one subscribes to Berkeley's metaphysic or not. A hallucination would be a creation of one's own mind rather than part of God's conception of the world. So it would lack certain characteristics of real things. Typically, one might appear to see something, but on approaching it and reaching out one's hand, fail to experience the associated appropriate tactile sensation.
This notion that whenever we look away, or close our eyes, objects spontaneously disappear under his metaphysic, is question begging. They appear to be assuming there is a material reality independent of our perceptions, but that also, paradoxically, its reality is dependent on whether we are looking at it or not. But, Berkeley thought the external world is entirely cashed out by our perceptions. So, it is incorrect to suppose that objects are constantly appearing and disappearing out of existence. That erroneously ascribes a position to him that he did not hold.
Incidentally, unlike the author, I wouldn't appeal to the limerick by Ronald Knox to resolve this alleged problem. Here's the limerick:
There was a young man who said “God
Must find it exceedingly odd
To think that the tree
Should continue to be
When there’s no one about in the quad”
“Dear Sir: Your astonishment’s odd;
I am always about in the quad.
And that’s why the tree
Will continue to be
Since observed by, Yours faithfully, God.”
First of all, as I have just said, there isn't any problem in the first place. Apart from that, I think it's simply not correct to say that Berkeley held that the external world exists by being observed by God. That, again, seems to imply that the external world has some type of prior existence, and it is God's perceptions of it that keeps it in existence. No, the external world is simply a conception within God's mind that he conveys to us.
To elucidate, our relationship to the external world might be compared to playing an online multiplayer computer game. The numerous players seem to see and interact within the same environment. The reality of objects in that environment -- such as a tree -- are represented by appropriately lit pixels. But, the pixels representing the tree, change depending on the perspective and distance the character controlled by the player is in relationship to that tree. And, of course, there won't be any pixels representing the tree should the character be turned so that he is facing away from it. As I said in my first blog post on Berkeley, this is explained by the fact that the computer game environment is governed by rules implemented by a computer programmer. Likewise, our external world exhibits uniformity due to physical laws with such physical laws simply being directly caused by God.
Extending the computer game analogy further, although the character we control is within the computer game environment, we ourselves certainly are not. We are sitting in our bedrooms or wherever we are when we play such games. Indeed, we could not be part of that game environment as our existence is not captured by lit pixels! In a similar manner, immaterialism holds we are not literally within the "material" world. What we label the "material" world is nothing but our perceptions, and it is therefore nonsensical to suppose we are part of it. Of course, our bodies are part of the "material" world i.e the those perceptions we identify with our bodies, but our selves and their conscious states are not part of that reality.
Support from Quantum Mechanics
I think Berkeley's metaphysic gets some support when we consider the implications of quantum mechanics. Or, at least it gets support in as much as it presents arguably insurmountable difficulties for those who suppose that reality is of a certain definite character considered entirely independent of one's perceptions or measurements.
The main difficulty is that subatomic particles, such as electrons, can either be particles or waves, but cannot simultaneously be both (just like an object cannot simultaneously be both a sphere and a cube). Yet, depending on the particular experiment, electrons behave either exactly like particles, or they behave exactly like waves. So what are they? Does their nature, their essence, change depending on how we measure/observe them? But that makes no sense since a thing's nature shouldn't change depending on how we observe it.
There is no problem under Berkeley's immaterialism though. Berkeley would have thought that a subatomic particle's reality (be it a photon, an electron or whatever) is purely a question of whether it plays a fruitful role in our scientific theories or not. It doesn't matter if, say, electrons exhibit particle like behaviour within one experimental context and wavelike behaviour under another since the reality of an electron cannot be abstracted from our perceptions/measurements of them. All that matters is that nature exhibits regularities that we can mathematically describe. In this regard, quantum mechanics is a runaway success.
It's important to note that this doesn't amount to the denial of the existence of electrons and other subatomic particles any more than it is a denial of everyday macroscopic objects (see near the end of my first blog post on Berkeley).