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[1] 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[2]. The manifest image of the world[3], 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.[4] All of the qualia, ‘what it’s like to be’[5] 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[6]) 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.

⊨(P → Q) ∨ (Q → R)

Try throwing that into English. Here’s a reading using some propositions I just came up with:

“I’ll die if I’m immortal, or I’ll live if I die.”

Obviously, neither of those are the case. But this formula, (P → Q) ∨ (Q → R), is both provable and self-implied in classical propositional logic.

Here’s a syntactic proof by means of natural deduction using some basic rules of inference:

Ok, there are definitely shorter proofs, especially if you let other equivalencies into your rules of inference, but this spells out quite a bit and gets the job done.

So, ⊢ (P → Q) ∨ (Q → R)

The reason this is provable, despite being formally contingent in English, is that the material conditional, →, does not represent the English “if/then”. If you’ve taken a course in logic, you’ve probably heard this. The reason is that the truth conditions for the material conditional are a little different. Specifically, a formula of the form (α → β) is true on every valuation except those that make α true and β false.

Look at our formula again. It states, semantically, that either (P → Q) is true, or (Q → R) is. If (P → Q) happens to be true, then the whole formula is true, so that case is covered right off the bat.

If (P → Q) is false? Well, there’s only one valuation that makes (P → Q) false — the valuation where P is true, and Q is false. So, if (P → Q) is false, then Q is false as well. But, if Q is false, then (Q → R) is true, because (Q → R) can only be false when Q is true and R is false. If Q is false, then (Q → R) is true.

Going the other way, if (Q → R) is false, then Q is true and (P → Q) can’t be false, because it can only be false if Q is false.

As a result, whenever (P → Q) is false, (Q → R) is guaranteed to be true, and vice versa. So, (P → Q) ∨ (Q → R) is a tautology.

My point here is just a friendly reminder: be careful with translation. Expressing the English “if/then” with the material conditional (→) may sometimes get you into trouble.

One of the major objections to the idea that time travel is possible is the apparent fact that we haven’t been visited by tourists from the future.  If travel to the past is possible, it’s likely that future historians may be tempted to take advantage of it, that terrorists or criminals may travel back in time to alter history, that someone would go back to visit their ancestor… and that, with those and so many other possible motives, it’s unlikely that the technology would never be used.

The most obvious answer to why we haven’t seen travellers from the future is that backwards time travel is either impossible or never gets invented.  Maybe humanity dies out before inventing it, for example.  But obviously we don’t want humanity to die out, and time travel is too cool to go uninvented, so what are we left with?

Fear not… all of your twisted sci-fi dreams may yet come to pass.  Here are some possible explanations for why we haven’t met any time tourists yet…

We’re living in a timeline that will be erased by time travel.

I’m not sure this one makes sense, because if this timeline, before time travel’s initial invention, will be overwritten by the consequences of time travel, it would seem that we wouldn’t exist as we do today — that if time travel can exist, it must create some kind of self-consistency, resulting in a timeline in which the timeline created by time travel produces time travel at some point after the time travellers first arrive.  Thus, there wouldn’t exist a timeline that wasn’t effected by time travel.  However, at least one initial version of the timeline has to have existed before time travel was invented the first time.  Maybe that’s this one, and somehow we’re to be forced out of existence when time travel exists.

I don’t like this option, because at worst we will have the totality of our existence erased, and at best, it just sucks.  Let’s try to do better.

Backward time travel has occurred, or does occur, but we’re unaware of it.

This one has a few possible options.  It may be that time travel has occurred but we’re unaware of it, because the resulting timeline has resulted in the human population, or at least most of it, lacking the knowledge of its occurrence.

One scenario to this effect is that, for reasons unknown, nobody goes back this far.  Perhaps the time travellers have yet to arrive — they’re more interested in seeing 2012, or 3009, than 2009 or earlier.  Maybe the human experience gets better, and they all think that experiencing our epoch would be torture.  Not the strongest of these options but one of the possible ones.

If time travellers have gone back this far, or further, perhaps it wasn’t documented, or very few people are aware of it.  Maybe tourists have only gone back to times in prehistory for scientific study, but wouldn’t risk contaminating human culture.  Or, perhaps the only travelers operated in secret, using technology or mundane disguise to conceal themselves, not revealing the fact that they were from the future.  (Perhaps you are the descendant of someone not born yet!)

If you want more of a stretch, perhaps the simulation argument is correct, and that its third possible conclusion is true.  This would mean that we are almost certainly living in a computer simulation.  Think The Matrix.  Maybe we’re living in a simulation in which time travel is either restricted or not occurring, but in the external “real” world, of which we are unaware, time travel occurs.

Time travel occurs between, or creates, multiple universes

Some current theories and models in physics, including string theory and some interpretations of quantum mechanics, suggest that our universe may be one of many.  The many-worlds interpretation of quantum mechanics, in particular, supports the idea that all possible outcomes of random events (and decisions) may occur in separate, ever-branching universes.

If this is true, we may simply be living in a universe that has not ye been, or won’t be, influenced by time travel.  It may be that our universe is the preservation of the original time in the first proposed scenario above — that when time travel occurs, the traveller does not, in fact, travel back to their proper past, but to a copy of it.  Time travel itself may spawn new universes, creating a new branch in the same way that other decisions would.

You can’t travel back to a time before time travel was invented

It may be that time travel requires some special conditions at the destination end.  In other words, you can only travel to a location in space-time that is ready to receive time travelers.

Stephen Hawking was one to propose this option.  He suggested that backwards time travel may require a special distortion or warping of space-time in the spatial location where the travel occurs, and as a result, time travellers won’t be able to arrive in a time where those conditions haven’t been created yet.

The movie Primer, which is one of my favourite movies, presents this option.  In the movie, the characters create a time machine, which is essentially a box in which time flows backwards.  They turn on the box, wait for a certain amount of time, climb into the box, and then wait for the same amount of time, emerging from the box at the point in which it was first turned on.  If that’s the only way time travel works, the limitations are severe.  It would prevent travel over long periods of time, unless the people in the time machine could be put into stasis, because you would need to exist within the machine for as long as the period of time you wish to travel.  And, of course, it means you can’t travel back to any time before the time machine was turned on.

So there you have it — four general reasons why time travel may still be possible despite the fact that we haven’t, to our knowledge, witnessed visitors from the future.  I like the fourth option best, as it seems to make the most sense and grants the most security to our timeline — until we invent time travel, that is.