Idealism is the thesis that the world is mind-dependent. In particular, the things we call material objects are dependent on the mind for their existence: to be is to be perceived. Realism is the thesis that the world is mind-independent, so that material objects can exist whether perceived or not. Here is a simple argument against idealism and in favor of realism: the mind depends on the brain, which is a material object, but no material object can depend for its existence on another material object; therefore, material objects cannot depend for their existence on minds. If material objects depended on minds for their existence, then they would depend on brains for their existence, because minds depend on brains; but brains are material objects, which would imply that other material objects depend for their existence on these material objects, but material objects never depend on other material objects for their existence; so material objects cannot be mind-dependent, and idealism is false. I think this is a valid argument, but it may be thought to beg the question: for can’t we say that brains also are mind-dependent? But suppose that were so: it would imply that the being of brains consists in their being perceived—but those very perceptions would themselves depend for their existence on a brain, and hence would call for a material object to exist that is not itself merely a perception. Perceptions of brains need brains too. Moreover, suppose that we had never perceived a brain and had no knowledge of the existence of brains: there would then be no perception of a brain for a brain’s existence to be dependent on. No, the existence of brains cannot be a matter of the existence of perceptions of brains; the existence of brains is an objective matter that is necessary in order for minds to exist. But since material objects can never depend for their existence on the existence of another material object, including a brain, they cannot depend for their existence on minds. The doctrine of “cerebralism” has zero credibility—that is, the doctrine that the entire material world depends on the existence of brains. No one supposes that tables and chairs, trees and mountains, depend on brains. That would not be idealism but a peculiar form of materialism—everything material is really one type of material thing. This is not the identification of reality with appearance, as idealism maintains, but the identification of reality in general with brain reality in particular (neurons, blood, brain chemicals).
Why don’t we see this point, thus rendering ourselves immune to idealism? The reason is that the dependence of mind on brain is opaque to us: we don’t have a grasp of how the mind depends on the brain, though we know that it does. Thus we think we can conceive of a universe in which minds exist but brains don’t, which contradicts the thesis of necessary brain dependence. But such intuitions are immediately suspect stemming as they do from ignorance of the mind-brain nexus. Not that the brain must be just as we conceive it, but there has to be something to do the job of the brain even if it isn’t exactly as we conceive this thing now (even Berkeley saw the necessity for finite and infinite spirits in addition to the ideas that exist in them). The fact is that (barring skepticism) we have discovered empirically that brains are the basis and sine qua non of minds, so that any dependence on the mind is also a dependence on the brain: but brains cannot be what material objects in general depend on, since material objects don’t depend for their existence on other material objects.
Note that the idealist claim is never that the world depends on someone’s mind (unless that someone is God); it is the thesis that my mind is what the world depends on. The idealist never says that the world depends for its existence on (say) the mind of Justin Bieber: why him, we might wonder. But we are easily seduced into thinking that our own mind might be the one on which everything depends (the world is my world). Why? Because of a certain kind of egocentrism: we don’t tend to see ourselves as merely one subject among many. Once I see myself as something in the world along with other subjects, I see that the world as a whole cannot depend on me in particular—why me and not him? Putting this together with the point about brain-dependence, why should reality depend upon my brain, which is just one brain among many (many billions in fact)? Why should this particular material object (the one in this head) be the font of all being? My brain does not enjoy this momentous privilege, and neither does my mind. In the case of God we are more inclined to accept that God’s mind could be the basis of all being, but that is because we don’t tend to see him as merely one person among many, with no more power to generate reality than anyone else; also, we are far hazier about the kind of existence God possesses. But putting aside this kind of theistic idealism and sticking to the usual secular kind, there has to be a question about whose mind is doing the ontological work; and Colin McGinn’s mind is no more privileged in this respect than Justin Bieber’s mind (rather less so). I am just one subject among countless others, and my brain is one among a multitude of brains (cf. my kidneys), so reality as a whole can’t depend on my mind or my brain. I am something in the world; it is not in me. We only fail to see this because of a stubborn (and callow) egocentrism that insists on putting ourselves at the center of things.
Once we see that our minds depend on our brains, and that our brains are just material objects among other material objects, the appeal of idealism evaporates. The world can never reduce to my world, i.e. how it seems subjectively to me. There must be a reality external to my experience (starting with the brain). This would not need arguing if the dependence of mind on brain were written into our everyday lived experience, but that would imply that the mind-body problem has a readily accessible solution. In a way, the mind-body problem is at the root of idealism: a solution to the former would do away with the latter.
 Spatially separate from them: of course, objects can depend for their existence on their parts. But it is not plausible to suppose that ordinary material objects have brains as parts. The general principle I am relying on is the standard notion of a material substance—that which does not depend on other substances for its existence (“self-subsistent”).
 I don’t wish to suggest that idealism is not a respectable philosophical position; indeed, I think that the debate between idealism and realism is one of philosophy’s deepest questions. I am simply pointing out its connection to the mind-body problem, also one of philosophy’s deepest questions. Idealism is as attractive as the mind-body problem is hard.