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The Metaphysics of Mortals: Cody Gilmore

By Andrew McCullough – In a recent paper, Associate Professor of Philosophy Cody Gilmore distinguishes between “personal” and “external” time, exploring immortality, time travel, and the philosophy of physics in the process.

How common is it for philosophers today to incorporate advanced theories of physics into their work?

There is a big and flourishing sub-field in philosophy that is entirely devoted to this: philosophy of physics. Extensive training in physics is a pre-requisite for philosophers who want to specialize in philosophy of physics; there is no sharp line between physics itself and the philosophy thereof. I work in a neighboring sub-field of philosophy, metaphysics (‘mainstream analytic metaphysics’), which is less focused on interpreting or applying current theories in physics and more focused on addressing a priori questions about fundamental categories (objects, events, properties, relations, facts) and on sharpening the ‘philosopher’s toolkit’ of general-purpose notions (existence, identity, truth, causation, parthood, location, necessity, essence, dependence, etc.). 

Training in physics is not a pre-requisite for specializing in metaphysics (fortunately for me!), but it’s not uncommon for people in mainstream metaphysics, or any area of philosophy, to be conversant in some aspects of physics, or to have majored in it in college. A bunch of people in my department know way more physics than I do. At my first academic job, one of my philosophy colleagues, Andrew Newman, had a PhD in physics in addition to his PhD in philosophy, though he works in mainstream metaphysics as I do, not philosophy of physics. But people in analytic philosophy outside of philosophy of physics tend to be very cautious about invoking physics in their work, maybe to a fault.   

Your recent paper “The Metaphysics of Mortals: Death, Immortality, and Personal Time” examines the nature of death and immortality using a framework that distinguishes “personal time” from “external time”. It is interesting to consider separate frameworks for personal and external time, given that relativity is all about how time (and space) must be variable to accommodate the invariant speed of light. In the paper, it seems you invoke the distinction as a means to amend a prior philosophical argument that you call ‘The Cessation Thesis. This is an attempt to describe what it means to die at a specific time—simply put, it states that “to die at a time is to cease to be alive at that time”. How did you come to the original Cessation Thesis?

‘The Cessation Thesis’ was just my name for what Fred Feldman called the standard analysis of death in his 1992 book Confrontations with the Reaper. The goal is to say, in as straightforward and literal-minded a way as possible, what it is for a thing to die. This can be done by filling in the blank in ‘x dies at t if and only if _____’. You start with the simplest remotely plausible proposal you can think of, produce a counterexample, modify the proposal to handle the counterexample, and repeat. Philosophers call it ‘Chisholming’ after the philosopher Roderick Chisholm, who spent a lot of time applying that procedure to various philosophically interesting concepts. Many contemporary philosophers find it annoying and tedious, or even bankrupt, but I think there’s a place for it, and I’m mostly a fan. I like to have every minute twist and turn of an argument brought out into the open for everyone to see.

In his dialogue Phaedo, Plato has Socrates say that for a thing to die is for its soul to separate from its body. For a materialist like me who doesn’t believe in souls, this is a non-starter. Fortunately, there’s an even simpler view that can be used as a starting point: for a thing to die is just for it to cease to be alive. That’s the Cessation Thesis. 

To some ears it will sound trivial or somehow circular. It is tempting to say, ‘Of course to die is to cease to be alive—but what we want to know is what it is to be alive!’ But as Feldman showed, this is too hasty. Even without a precise account of what it is to be alive, we can show that dying is not the same thing as ceasing to be alive. Feldman gives two examples that make the point. 

First, there is terminal biological fission. When an amoeba divides into two new amoebas, the original amoeba ceases to exist—and so ceases to be alive—but it would be false to say that the original amoeba dies. Second, there is suspended animation, or what biologists sometimes call cryptobiosis. When a tardigrade freezes or dries out and enters cryptobiosis, its metabolism and other life-functions shut down,  which makes it plausible that it ceases to be alive. But it can return to life again easily, which makes it plausible that it doesn’t die or become dead at the relevant time. This view of cryptobiosis is also influentially defended, independently, by UC Davis biologist James Clegg in his 2001 article “Cryptobiosis—a peculiar state of biological organization”. Clegg, like Feldman, says that there are three mutually exclusive states that an organism can be in: it can be alive, it can be cryptobiotic, or it can be dead. I think that’s right.   

So, ceasing to be alive is not sufficient for dying: things can cease to be alive without dying, either by undergoing certain forms of biological fission or by entering cryptiobiosis. The Cessation Thesis fails. Can we fix it? This is where the really interesting and imaginative Chisholming begins—but for that you’ll have to read Feldman’s book or, for newer iterations, a couple of my papers.

You present “external time” as a continuous series of instants in which biological changes are happening at a certain rate. In contrast, “personal time” is the continuous series of moments that one would experience as normal time, no matter how long or short it is in comparison to “external time”. Effectively, one could experience “personal time” very slowly or much faster than the passage of “external time”. This leads to an interesting perspective on immortality, as one could theoretically experience personal time that goes on forever (in relation to external time). Have I got that essentially correct? 

The contrast between external time and personal time comes from David Lewis in his 1976 paper “The Paradoxes of Time Travel”. He argues that the famous time travel puzzles are not really contradictions and do not (contrary to what many philosophers and physicists have thought) show that backward time travel is impossible or that it would require parallel universes or an extra temporal dimension. It would ‘only’ require locally backward causation or some version of globally circular time. 

As Lewis uses the terms, external time is just time itself, whereas the personal time of a particular person or other object is, to a first approximation, just what would be measured by a clock carried along with the object. (Those who know some physics may be tempted to interpret Lewis’s notion of personal time as relativistic proper time. This turns out not to be right, but the two notions can do some of the same work in making sense of certain kinds of time travel.) In the H.G. Wells novel, The Time Machine, when the machine is traveling into the future, the personal times of its occupants fall out of step with external time. Viewed from outside the machine, their biological processes appear to ‘slow down’. They might make a journey of 1,000,000 years into the future, and yet they might have undergone just one minute’s worth of biological changes: just 70 heartbeats, four breaths of air, and so on. In that case, their journey takes 1,000,000 years of external time, but only one minute of their personal time. That’s the basic idea, though Lewis ends up offering a more sophisticated account of personal time.  

You also describe how “personal time” could occur in non-contiguous points within “external time” – effectively, jumping through time. So, this is time travel in the sense that we more commonly think about—an individual’s continuous “personal time” occurring in disconnected segments of “external time”. Do you believe this is possible for a human—to live part of their life and suddenly jump to another point in external time? Or do you view these principles as theoretically possible, maybe for other life forms, but only using humans as descriptive examples in the paper?

Philosophers like to distinguish between nomic possibility (roughly, what’s permitted by the laws of physics) and absolute possibility (roughly, what’s possible in the broadest sense, where this requires logical consistency at a minimum). For example, getting a bullet to travel faster than light is not nomically possible, but it would seem to be possible in the absolute sense—there’s nothing internally contradictory about the scenario.

When philosophers give an account of something, we typically don’t intend our account to be restricted to situations in which the actual laws of physics still hold; rather, we typically want our account to handle every case that is absolutely possible, even cases in which different laws of physics apply. In Lewis’s words, we want our accounts to hold ‘not only for the cases that arise in real life, but for all possible problem cases as well’.

At least that was my policy when I was working on successors to the Cessation Thesis. I considered, among other things, cases in which an organism has a temporally gappy career; it ‘jumps forward’ in time.

Some cases like that might be nomically possible. Maybe artifacts like watches sometimes exist intermittently: maybe a watch goes out of existence when it is disassembled and comes back into existence when those same parts are reassembled. If so, then maybe something similar could happen to a very simple organism, even in a universe governed by the actual laws of physics. Maybe something like this could even happen to a human body. I don’t know.

Finally, there is the view—which I do not endorse—that we (people) go wherever our memories and other psychological states go, so that if my body gave out and my memories, etc., were uploaded into a computer that could, so to speak, ‘run my psychology’, I would continue to exist and would inhabit the computer. I might then exist intermittently—maybe I would exist only when the computer was running the program associated with my psychology. Again, these are various reasons one might have for thinking that it’s nomically possible for a person or organism to exist intermittently, i.e., to be temporally gappy.

But in the paper, I also touched on even more exotic cases, e.g., cases in which an organism miraculously jumps forward in time, just popping out of existence at one time and popping into existence out of thin air at a much later time. I don’t think cases like this are nomically possible. For all practical purposes, they can be ignored.
But it can be interesting to figure out how such cases, if they are possible in the absolute sense, bear on the attempt to say what it is for a thing to die. Though even in that context it is wise to avoid resting too much weight on them.

Anyway, I suggested that if a living thing did this—if it popped out of existence at the end of 2016 and popped back into existence at the beginning of 2018—that would be another counterexample to the Cessation Thesis. In that case, the thing would cease to be alive at the end of 2016, but it wouldn’t die then; instead, it would just jump forward in time. Likewise, if the organism, without jumping, simply ‘reversed direction in time’ at the end of 2016,  as an electron does in certain interpretations of electron-positron interactions. In such a case, the organism would cease to be alive at the given moment (in the sense that it would be alive throughout some interval leading up to that moment but it would not be alive, or even exist, throughout any interval beginning at that moment), but it wouldn’t die then.

The ancient Greek philosopher Alcmaeon discusses scenarios involving circular time and is reported to have said that humans die simply because they are ‘not able to join their beginning to their end’, i.e., to make their entire lives into loops. In this vein, I point out that if time formed a circle, and if a person’s life coiled around the circle, say, one and a half times, then the person’s life would have a beginning and an end—and the person would die at the end—but at each instant of external time, the person would be alive, and so in that sense the person would never cease to be alive. So, this is yet another counterexample to the Cessation Thesis.

As you may be able to guess, I eventually repair the Cessation Thesis by bringing in personal time. That much is obvious. The real action is at the level of deciding between inequivalent accounts framed in terms of personal time, and in deciding what role should be played by external time. It turns out that there are some strong and non-obvious arguments for favoring a certain one of these ways of doing it. Hitting on arguments like these is what I get excited about when I’m working on a paper. 

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