Existence and Consciousness
The idealist sees an essential connection between existence and consciousness: there is no existence where there is no consciousness. Can we make anything of this thought? Suppose an otherwise empty region of space contains an instance of consciousness, say an experience or thought; then we can rightly say that something exists in that region. Consciousness is the kind of thing that confers existence (this is the root of the Cogito). The thing that exists might be said to be the conscious state itself or its bearer (the subject of consciousness). Moreover, this thing is a concrete empirical existent not a merely abstract one. Consciousness is a paradigm of existence; it leaves no doubt of existence. Contrast consciousness with matter: suppose we stipulate that within a certain region of space there is extension, i.e. length, breadth, height, size, shape. Is it an immediate consequence of this that something exists in that space additional to the space itself? No, because space itself has extension, i.e. geometrical properties. It doesn’t follow from the instantiation of geometric properties that space contains an actual concrete empirical thing. Something needs to be added—something concrete. To put it another way, the concept of extension is a mathematical concept, so if matter is defined in terms of extension, we won’t be able to derive concrete existence from it. There is no analogue of the Cogito as follows: “X has extension, therefore X is a material existent”—not if matter is a concrete empirical thing. Thus consciousness brings existence with it while matter (defined as extension) doesn’t: we can’t get our ordinary concept of a material thing out of mere extension. It would be different if we could supplement extension with substance in the Scholastic sense, but that notion is not available owing to unintelligibility (certainly anathema to Descartes). Intuitively, we have no account of the distinction between empty space and what materially occupies it. We thus don’t know what the existence of matter consists in: the concept of extension (geometry) leaves it schematic, abstract, merely mathematical. To be sure, matter has extension, but what we don’t know is the nature of the thing that has it; whereas we do know the nature of consciousness, i.e. what it is that exists when consciousness exists. We have a real conception of mental existence, but we don’t have a real conception of physical existence. In practice we fill out the abstract notion of extension with the concepts derived from our perception of material things, e.g. color, but these have a mental origin, so they can’t be what the objective existence of matter consists in. The suspicion is that when we speak of the existence of material things we know not whereof we speak.
This is where the idealist plants his flag: the only way to explicate the nature of material existence is to borrow from mental existence—material things are really mental in nature. Then we will understand how material things can have concrete existence, just like mental things—they are mental things. They might be sense impressions in human minds or ideas in the mind of God or a special kind of primitive consciousness found in so-called material reality (panpsychism and its ilk). According to each theory, material things turn out to have the kind of existence possessed by mental things. Existence is thus univocal and uniform: all of reality exists in the same way. It is not that minds exist in virtue of one kind of property and bodies exist in virtue of another kind—existence is always mental. To exist is to be conscious in some shape or form. The price of rejecting idealism is to render the existence of bodies problematic, not to say impossible. For what else could their existence consist in? You might try saying that bodies have properties other than extension such as mass, charge and solidity: where these properties are instantiated there must be existence. But these are merely dispositional properties, unlike extension, and so raise the question of what grounds them: what is the intrinsic nature of body? The existence of a thing cannot consist solely in its dispositional properties on pain of rendering it mere possibilia. Again, the contrast with consciousness is stark: in its case we do have a grasp of the intrinsic nature of the existent thing. The idealist insists on something analogous in the case of material bodies, and it is obscure what that might be if it is not more consciousness.
There is a possible view that can block the idealist’s argument, namely that matter possesses an unknown type of intrinsic property that plays the existence-conferring role of consciousness without being consciousness.Call this property M: then we can say that bodies exist in virtue of instantiating M, where M is not identical with C(consciousness). This seems like a logically available position, but one can appreciate why the idealist will jib at it: why postulate such an unknown property when we have a well-known property that can demonstrably do the job of securing concrete existence? Isn’t the idealist position less hand waving, more parsimonious, more intellectually satisfying? Why go noumenal and mysterian when idealism offers such a nice uniform theory? Idealism tells us exactly what existence consists in, intelligibly and invariably, so why speculate about hypothetical unknown properties? Without it we are left with no positive account of what physical existence amounts to—a mere I-know-not-what.
Historically, idealism arose from Descartes and Newton’s mathematical conception of the material world: there was a distinct danger that the material universe might disappear in a puff off mathematical smoke. Indeed, it wasn’t long before theorists began doubting the concrete existence of material things and regarding such talk in an instrumental fashion. If matter is really geometry, we might as well regard talk of it as so much applied mathematics. Physics seemed to take the substance out of the world—it took the body out of body. But idealism resisted this etiolating tendency: it allowed us to recover our sense of the concrete reality of body, albeit in mental form. Before mathematical physics arrived, Aristotle’s teleological physics allowed the concept of purpose to fill out the theory of motion; and purpose could plausibly be supposed to guarantee concrete existence—what has purpose must exist. But once this is banished and classical mechanism is allowed to fix the nature of matter (bloodless extension), a gap opens up in our conception of matter, a gap that threatens its very existence. The contrast between mind and matter becomes unsustainable and matter loses its grip on concrete reality. Thus Berkeley meets fertile ground for saving bodies from evaporating into abstract posits. At least with Berkeley we know what it is for bodies to exist! Idealism allows matter to have the kind of being we understand—the kind possessed by our own minds. Berkeley’s world is a world of complete intelligible existence, whereas Descartes’ world is a world of intelligible existence (the mind) alongside a world of unintelligible existence or faux existence (matter). The fundamental problem is that extension by itself is not sufficient to deliver concrete material being. Nor is it clear that anything in contemporary physics is sufficient either, which is why the physical world is apt to appear theoretically ethereal. In order to deliver concrete reality our conception of matter needs beefing up, and idealism offers itself as the only viable way to do that. For the idealist, existence without consciousness is no existence at all, because in the end consciousness is the only intelligible form of existence there is.
 Surely part of the reason we have trouble with Platonism in mathematics is that we can’t form a clear conception of what mathematical existence would be; we feel we are taking it on faith. Numbers are quite unlike episodes of consciousness, in which existence is carried on their face: hence the attraction of mentalist theories of numbers and nominalism generally.
 Let me emphasize the alternative—that there are other forms of existence that are unintelligible to us. This position is by no means absurd. The question then becomes whether idealism faces insurmountable problems (I won’t discuss this here).