Even if Berkeley is right that we cannot, without contradicting ourselves, conceive of an unconceived tree, this does not imply either that (a) we do not in fact conceive of such things (after all, we can conceive of impossible things like the largest prime number), or that (b) there could not be an unconceived tree (the link between imaginability and possibility is not nearly so strong as Berkeley supposes).
But one can very well say that we do not, in fact, conceive of impossible things like the 'largest prime number'; we simply use the words 'largest prime number'. And there is nothing in Berkeley's theory to prevent saying this; in part, because it is precisely along the lines of the sort of thing Berkeley says. Merely because you use words meaningfully (in a linguistic sense), it does not mean you have a notion to correspond with those words (nor does it mean that you've said something consistent). And on (b) it isn't actually very clear what one can mean by saying that I can conceive the existence of a tree whose existence I cannot conceive. The same point arises with another response to Berkeley that's discussed:
Although his argument purports to show that I cannot conceive of any particular unconceived tree, it seems to leave open the possibility that I can have the thought that there is some tree that is unconceived. That is, I may be able to have thoughts about unseen trees that are descriptive, rather than directly referential.
Consider the following story:
Once upon a time, there was a universe with nothing in it except for a single tree. Since there were no people or thinkers or anything else in this universe, the tree just sat there for all of time, and no one ever saw it, or thought about it, or told a story about it. The end.
My story is boring and probably lacking in literary merit, but it is not incoherent. Since Berkeley’s argument – even when weakened with the quantifier response – claims that it is inconsistent, the response in question is insufficient to stop Berkeley from proving something absurd – that my story is conceptually confused.
But it isn't obviously absurd to say that the story is conceptually confused; that is, it isn't clear that the story it gives is coherent, and even if it is coherent, it isn't clear that its coherence is a problem for Berkeley. (It's coherent, of course, even on Berkeley's view, to say that no one has told a story about the tree about which I am currently telling a story; it would be incoherent, however, to say that I have a story about a tree about which there are no stories, simpliciter, and it is this that Berkeley would claim to be parallel to the case of matter. Such things make great fiction, but bad philosophy.)
The author's own suggestion is very interesting; but I'm not very clear why it's supposed to be plausible. I think of a tree does, prima facie, appear to imply A tree is thought of by me, because they seem to be saying simply the same thing. And if this is so, I'm not sure why the issue of the relation between the thinker and the thing is going to make any difference to Berkeley's argument. If I claim, "I have an idea of a tree of which I can have no idea," what generates the contradiction is not the relation between thinker and tree, but the contradiction in saying "I have an idea I do not have". It's like the old invisible pink unicorn example; one cannot see a unicorn one cannot see. And one needs no theory of seeing to recognize that this is so. Likewise, one needs no theory of thinking to see that we cannot think of things of which we cannot think.
My own view, only very roughly thought out, is that Berkeley is quite right: we cannot say that we conceive an unconceived thing, or perceive an unperceived thing, properly speaking, without contradicting ourselves. And I don't see that there is any problem with saying this; we cannot have ideas we cannot have, we cannot have perceptions we cannot have. Berkeley needs more for his idealism than just this, but he's right on this point.
[We do, of course, have a notion of not conceiving, because sometimes we don't conceive things. But this doesn't imply that we can ever conceive of something unconceived, properly speaking; it just means we know we are not always conceiving everything. When we do conceive things, they are not unconceived; when we are thinking about things, we are thinking about them. The things themselves, however, can still be in some sense independent of us, external to us, and continue to exist when we are not thinking about them; and Berkeley, in fact, accepts all three of these claims. In this argument he merely denies that we can ever have in mind the being of something that is properly speaking 'without the mind'; and his interpretation of the three claims does not treat the independent, external, continuing existence of things as 'without the mind'. In other words, he breaks down the internal/external dichotomy that undergirds external world skepticism.]