In his Mind and World, McDowell contrasts three positions in his concerns with spontaneity and intentional states in general. These are what he calls bald naturalism, rampant platonism and naturalized platonism, the last of which he defends. What McDowell calls ‘bald naturalism’ I will argue for, but I will refer to it more favourably, as ‘naturalism proper’.
The proper naturalist position counts the mind as part of the world, as McDowell wants to do. What a naturalist proper cannot coherently talk about is the Kantian subject. Instead, there are only objects. For those who would say that things are only objects in the sense that they are the object of some subject, then we may use the word ‘substance’, in the earlier Greek sense, to talk about physical things. Thus, the naturalist position is that everything is a substance; in Heidegger’s terminology, everything is essentially present-at-hand and though further supposed properties or features of substances (readiness-to-hand, the existence of Dasein, the mind, intentionality, subjectivity) are reducible to that raw substance. In contemporary physics, that substance is identified with particles, though the details may be somewhat more complex than that. The manifest image of the world, the normative space in which intentionality and morality are thought to lie, is composed of that same physical substance that makes up the brains of those who experience and project outwards.
So, for the naturalist proper, the space of reasons is not a realm outside of the natural or physical world, as it is in rampant platonism, nor is it an autonomous space within the world as it is in McDowell’s naturalized platonism. Rather, all rationality, normativity and intentionality is identical with brain states. There is no one space of reasons with which human beings have a connection; rather, each human being can be thought to have their own space of reasons. This space is not especially private, though. It can be seen physically by examination of the neurological states of the brain, and with the proper technology, those states may even be translatable into intelligible images. In another sense, it is not fully private, since glimpses into its content can be seen through behaviour.
‘Content’ needs qualification in that last remark. Intentional mental states are mental states that are about something else in the world. But they are neither a metaphysical link through some imaginary (or real) space to other substances which they are about, nor are they really representations of the substances they seem to be about. They are not the former because such space doesn’t exist in any important way. They are not the latter because, of course, the neurological features of our brain do not form a structure that mimics the outside world. Rather, they form a structure that allows us to have a mental state (which is identical with that brain state) such that we think we have a representation of the substance we are thinking about. So, when I close my eyes and picture the CN Tower, I do not obtain or summon some representation of that object. That object may be gone, moved, or different than I remember. Instead, my brain generates a certain arrangement such that it looks like the CN Tower to itself. All of the qualia, ‘what it’s like to be’ and private world phenomena that philosophers expect to find in a separate mental world are merely what the brain’s activity looks like from the point of view of that very brain.
Against the rampant platonist, the naturalist’s response is brief. Whatever attitudes one has towards the empirical observations with which one is presented, one does not survive without indulging in them. With a proper account of naturalism, any reason to favour a platonic account dissolves. Naturalism proper offers an explanation of intentionality, removes the “spookiness” (McDowell 92) of a separate space of reasons that McDowell complains of. We don’t need to add anything ontologically, and we have a hope to answer the how questions about mental states and intentionality through study of the brain. Spontaneity doesn’t need any explanation here, either, because it is not sui generis, if it’s counted as existing at all. We’re better off with the naturalist account.
Against McDowell’s naturalized platonism, the proper naturalist has to say that McDowell is wrong about naturalism. He argues against his ‘bald naturalist’, saying that “knowing one’s way around the space of reasons, the idea of responsiveness to rational relationships, cannot be reconstructed out of materials that are naturalistic” (McDowell 77). But the naturalist proper doesn’t want to reconstruct a space of reasons, really. The naturalist proper holds that all reasoning, all intentionality, all communication—the entire manifest image—takes place between physical substances across physical substances. The naturalist isn’t committed to reconstructing any intentional state beyond this because she doesn’t hold it to exist. What counts as intentionality for a naturalist proper is evidenced through behaviour, and it consists of a brain processing incoming information. The space of reasons, the finer points of language, art, morality, knowledge and so forth can be talked about as abstractions. They are useful in the processing that the brain must do; they result in digestible inputs. But the naturalist proper is ever-mindful that these are not things in themselves, but that they are always identical with and reducible to the states of the brain. The same holds of spontaneity. The naturalist counts human judgments to be determined (in a deterministic or indeterministic sense) by the physical processes that cause them. Thus, spontaneity can be said either to not really exist, or at least to be denied status as sui generis, as is the case with the other ideas McDowell wants to preserve. Where McDowell wants to say that human beings are free to choose their beliefs, to take their experiences and spontaneously form judgments about them, the proper naturalist denies this. For the naturalist proper, sometimes brain states will have the disposition to respond to the stimulus of a red bench and form the belief that there is a red bench. Sometimes, because of other beliefs, brain states will not have such a disposition and will not form such a belief. Whether or not this happens is not an act of spontaneity, but a function of the states of the brain, including the stimulus received and the beliefs already held.
By positing an autonomous space of reasons, and a second nature with which humans can access it, the naturalized platonist gains little in the way of explanation and much in the way of recalcitrant philosophical questions. McDowell thinks that he gets out of the “threat of supernaturalism” (78) by saying that scientific advancements don’t clarify nature as a whole, but only the realm of law. This step does not work; it only redefines “nature” to include what the naturalist counts as supernatural. McDowell renames nature as the realm of law, says that the platonic heaven, the space of reasons, also exist, and says that there’s this new thing, now called nature, previously called existence, which encompasses all of them. The connection between the realms of laws and reasons is as sketchy as ever, and it is only through proper naturalism of intentionality and spontaneity that the confusion is finally dissolved.
 String theories and wave-particle duality in physics play a role here, but they are beside the point. Scientific skepticism about what substances are physically is important. Equally important is the fact that extra-physical phenomena haven’t turned up at all. That is, the naturalist account is the best lead we have.