Idealism is a kind of boogey-man for philosophers of an analytic stripe. Indeed, when polled as recently as 2009, the PhilPapers survey found that only %4.3 of philosophers are idealists about the external world, and presumably the objects located therein. When it comes to idealism about entities in the external world, there is a happy rejection of the idealism that predominated the United Kingdom and European continent in the latter part of the 19th century. This post is not meant to take away from the perceived victory of the realists, but rather trouble some of the quick dismissals that realists rely upon in order to present idealism as being a metaphysical nonstarter.
Kant's engagement with the idealists and realists before him should make it apparent that his transcendental idealism demands more attention from analytic philosophers than it is often given, but there are a couple of reasons why I think he was left to the curb with the rest of the German idealists that go beyond the caricatures. The first is idealism is clumsy at best when discussing psychological and mental phenomena. When we say that the mind is responsible for crafting the objects that we perceive within external reality, there is a conflation between the crafting of physical entities as what's fundamental to their existence and the conjuring of objects that is akin to hallucinatory experiences. The second is in the continuing theoretical challenges from particle physics to the notion that reality has to be conceivable in order to be understood. Kant may be redeemed within quantum mechanics, but there is not an obvious answer as to how that would happen.
This entry is not about repudiating any the above concerns. In fact, I would say that the two points made above are ongoing concerns for the %4.3. With that as the opening, I will spend the rest of the post going over two grounds for dismissal that are unmotivated from realist philosophers.
Firstly is the mind-independent criteria that realists believe makes any alternatives unattractive. This concern is caricatured by Bertrand Russell's remark about hoping to assert the moon's existence even while he is not there to look at it. The notion that the operations of the mind are what brings about the existence or the properties of objects is no longer much in the running for the question of fundamental entities, insofar as the category of a fundamental entity vanishes within a metaphysics that requires minds to be operating in order to bring about the existence of objects at all.
But this concern is a non-issue for idealists of a particular stripe. Leibniz believing that all phenomena reduce to some kind of mental entity is a good way to think about a different kind of contradiction surrounding mind-independence. It'll be the case that phenomena are existing without depending on other entities, namely that chairs and the number three do not need my brain to be cogitating on them in order to exist, but the table and the chair reduce to some mental phenomena, or their particular monads. In this way, by rejecting materialism, then we have the difficult task of trying to short hand precisely what's going on when a philosopher believes that the fundamental constitutes of reality are mental, not that it is the mental that brings them about fundamental reality. If that last sentence seemed obtuse, I'll try it again. Fundamental things being mental phenomena does not mean the operations of the mind bring them about.
Secondly, a concern that realists have regarding the notion of mental phenomena is that it has a hard time distinguishing between physical processes and mental processes, which must be distinct if one is to explain the behaviors of animals and the other is used to explain the existence of certain entities. The problem with trying to understand the interaction between mental and physical phenomena is difficult, but the interaction problem is something metaphysicians of every stripe must answer to, not just idealists. Some realists have taken it so far that there is no mental phenomena what so ever, and that the only fundamental things there are physical entities. I will leave the concern here, but it should be understood that non-reductionism within the philosophy of mind is not a problem for just idealists, and therefore it should not be pinned as being exclusively in their territory.
Let me take on two issues for realists before I close.
The eliminativist project has always had two problems. One is it eliminates the category of knowledge as being something true about eliminativism. If knowledge can be reduced to it's neural correlation, then that the ability to say truth things about reality now corresponds with neuron firings rather than just their truth value. The reasons for this being concerning should be self-explanatory, but I will belabor the point. Eliminativism rests on a circularity that it is trying to avoid, namely, that mental states are reducible to their physical correlates, and in this case, there are no mental entities; just the physical stuff doing what it does. But then that would have to mean that even statements about eliminitavism are merely their physical correlates, such that we cannot speak about physical processes with it merely being the state of physical processes. This has neutered our understanding of truth-making claims to the degree that I have no longer captured the notion of truth that most of us operate with, namely, speaking about things in a way that accurately represents them.
Platonic realists about particular properties, in particular the theory of forms, will have a hard time of account for the plurality of entities. Presumably, infinitely many properties must have a single origin, but their difference from their creator cannot be such such that they are radically separated from her to the degree that their natures are not in some way similar. Idealists have an easy time accounting for this similarity by appealing to the nature of these entities and the creator as being things that are fundamentally mental, rather than fundamentally embodied or distinct. Realism then transforms into a kind of monism about the world which still must account for the perceiving of distinct phenomena. Either way the direction goes, a pluralist explaining things reduce to one thing, or a monist trying to account for everything, idealism versus realism is not where the debate lies.