Stephany Garcia

Illustrated by Khadeeja Safdar

(i)

Do you think that a world in which 2+2=5 exists?

If you’re asking me whether I think the sum of two and two might have been five, then the answer is clearly ‘No.’ It is a necessary truth that that sum is four. People sometimes get confused about this because they think: “Well, the numeral ‘5’ only conventionally refers to the number five, so it’s possible that it might have named the number four, and then the sentence ‘2+2=5’ would have been true.” That’s correct. But what’s being imagined there is not a world in which the sum of two and two is five. Rather it’s a world in which the sentence ‘2+2=5’ just happens to be true because it means that two plus two is four.

On the other hand maybe you’re asking me what I think about the metaphysics of modality. I’m not a modal realist; I don’t believe there are possible worlds other than the actual world, so I don’t think that any worlds exist in which actual falsehoods are true. In my opinion that’s the wrong way to cash out talk about possibility. I think that Al Gore might have been president, had things gone differently from the way they actually went, but I don’t believe that that modal claim is made true by the existence of a possible world in which Gore was president. Possible worlds are a very useful heuristic device for thinking about the semantics of modal logic, but we shouldn’t take them seriously ontologically.

Few metaphysicians do accept modal realism, except, famously, my teacher David Lewis. And he really did believe in them! I remember sitting next to him at a dinner at Princeton years ago when Scott Soames leaned across the table and whispered: “They’re really out there, aren’t they David?” Lewis had consumed a couple of pints of beer by that point, and maybe Soames thought that he’d catch him at a weak moment. No chance! David just beamed back at him and nodded vigorously.

(ii)

What applications do you think Decision Theory has to a person’s daily life?

I once stood on a street corner in San Francisco—in the rain—for nearly ten minutes in the company of five other decision theorists who had just attended a panel discussion at the West Coast APA Conference. We were trying to figure out which restaurant to go to...

I think people can easily get the wrong idea about decision theory. The theory says that in order to be rational we should always be calculating and maximizing expected utilities. Well, no. Like most people, I hardly ever think explicitly in those terms in daily life. At the heart of traditional decision theory is a representation theorem that says: if a person’s preferences over available options satisfy such-and-such axioms, then we can construe that person as a rational agent with well-defined utilities and degrees of belief who is acting so as to maximize expected utility. But that’s not to say that the person in question is using the theory as an explicit algorithm for deliberation. Philip Pettit has a helpful analogy here: keeping your balance as you walk down the street is, as it turns out, a matter of keeping the fluid in your ear canal level. But that’s certainly not what I’m thinking about when I’m trying to keep my balance, let alone when I’m just walking along the street.

That said, there’s a lot of interesting recent empirical work that reveals characteristic and systematic flaws in human decision-making. I’m thinking in particular of the work of Kahneman and Tversky and their followers. I think it can be very useful indeed, and of great practical importance in daily life, to be aware of the kinds of biases and mistakes to which one may be naturally prone in deliberation.

Here’s an example: Owain Evans, who was a major in this department a few years back, wrote a wonderful senior thesis on the “ratio heuristic”: (1) people are more likely to travel across town to save $10 on a $20 purchase than on a $200 purchase; and (2) people have strong intuitions to prefer humanitarian interventions that save a larger proportion of lives, even when the absolute number of lives saved is no greater.

Very imaginatively, Owain wondered whether this tendency might be explained by recent neurophysiological research on the mental representation of number. The gist of this work is that humans seem to share with other non-human animals a very basic sort of continuous or analog system of representation of the discrete natural numbers. We appear to store information about number in a continuous manner, rather than digitally, and that would tend to make ratio comparisons more salient to us than comparisons of absolute difference.

(iii)

Do you consider yourself a philosopher king, in Platonic terms?

Not at all. I would be a very bad king, and I have never harbored any political aspirations of any kind. I can hardly imagine a role I would be less suited to, except perhaps neurosurgeon. I would be a truly terrible neurosurgeon. The nice thing about philosophy is that nobody dies when you make a mistake.

I do think, however, that the rather depressing cacophony that passes for political debate in this country might benefit from the input of more people with philosophical training. One skill that philosophy teaches is the ability to see what is, and what isn’t, a good reason for believing something. I think the level of political discourse might be improved by the involvement of people who realize, for example, that just because one agrees with the conclusion of an argument doesn’t mean one should think that the argument is a good one.

(iv)

What has been the most real dream experience you’ve ever had?

I find that I very rarely remember my dreams, and those I do recall on waking tend to be pretty strange. Most real? Well, I once dreamt the solution to a mathematical problem when I was a student. There was an undergraduate mathematics society at Sydney University that went by the happy acronym ‘SUMS’. They had an annual problem competition. I’d spent all evening struggling with a combinatorial problem and had gotten nowhere with it. Then I went to bed and woke up in the early hours of the morning having had a very vivid dream in which I had figured out the solution. I remember being very skeptical of this “dream solution” and I actually got out of bed and walked over to my desk and jotted down just the key step, so I wouldn’t forget it. Then in the morning, to my amazement, it worked! I was completely flabbergasted. I still didn’t win the competition though.

(v)

What would be the one thing you’d want to do if time travel were possible?

I suppose this is a sort of disappointing way to answer your question, but to tell you the truth, if I had the opportunity to time travel into the past, I would turn it down, and I would stay well away from the time machine.

I gave a talk on this here at Columbia recently called “The Perils of Time Travel.” There is reason to think that time travel to the past would be a very unpredictable and, in fact, a potentially extremely dangerous business. There’s a fascinating 1991 paper in Physical Review by Echeverria, Klinkhammer and Thorne that examines a computer model of a classical billiard ball sent off towards a wormhole on a trajectory that threatens to lead to a paradoxical collision with its earlier self, a collision that would prevent the ball from having entered the wormhole in the first place.

Now of course we know that no such thing can happen—time travelers can’t change the past—but what Echeverria et. al. discovered, to their surprise, is that there are always infinitely many ways that such a ball’s history can play out consistently. Most of these involve incredibly fine glancing self-collisions that change the ball’s course only very slightly, and not by enough for it to miss the wormhole. The worrying thing is that the laws of physics, even if they are assumed to be Newtonian, turn out to be radically indeterministic in such a setting. Deliberate or unconscious attempts to change the past will always fail, but there’s simply no saying what will prevent them from succeeding, and it might turn out to be something very unpleasant indeed.