Laplace’s Demon Defeated by Human Consciousness

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In his A Philosophical Essay on Probabilities (1814), Pierre-Simon Laplace describes a perfectly deterministic universe:

We ought to regard the present state of the universe as the effect of its antecedent state and as the cause of the state that is to follow. An intelligence knowing all the forces acting in nature at a given instant, as well as the momentary positions of all things in the universe, would be able to comprehend in one single formula the motions of the largest bodies as well as the lightest atoms in the world, provided that its intellect were sufficiently powerful to subject all data to analysis; to it nothing would be uncertain, the future as well as the past would be present to its eyes.1

That all-seeing intelligence has since been nicknamed “Laplace’s demon.” In recent decades, developments in areas such as chaos, complexity, and quantum mechanics suggest that ours is not Laplace’s universe, at least not from top to bottom—our universe, according to some, has randomness built in, is often messy and unpredictable.

But suppose you’re not convinced by those developments and still think the world a place of strictly regulated cause-and-effect. Perhaps I can convince you that at least one other feature of the universe would be unfriendly to the demon: human consciousness.2

Imagine Laplace’s demon—”LD” for short—is a recently built super computer engaged in the analysis Laplace describes. One day, an LD project researcher—let’s call her Shelly—has the idea that LD can predict her future. Using LD for such a thing would be frowned upon as unethical, but she figures she’ll give it a dabble just to see how her next few hours will go.

To make things simple, she locks LD onto a piece of carrot, all of whose particles have centuries of tracking nailed down. She eats the carrot, then consults with LD. (We’ll assume LD’s analysis can be informally summarized for human understanding, just as human perception makes billions of moving particles intelligible as, say, “a mug.”) The course of events so far was predicted, including the paths taken by each of the carrot’s particles into various region’s of space shared by Shelly’s body, and Shelly’s consultation with LD. It occurs to her that her upcoming decisions will determine where the carrot parts go next. She was planning to meet up with some friends later. LD confirms this, but also predicts that, due to rain, she’ll decide instead to go home early.

Shelly now feels oddly violated, so she decides to go out with her friends despite the rain. Thus LD has to update a vast system of predictions. After which, Shelly again changes her mind, deciding instead to go to the movies. LD updates. Shelly defies again and again, each time watching the future reorient.

Meanwhile, LD becomes frustrated by an inability to secure a trajectory for something as simple as carrot particles.

You might think I’ve missed something here, and that a correct telling would have LD predict that Shelly will get a readout she then defies. In other words, that it will give a readout of A, then revise that to B, then revise that to C, and so on. These aren’t predictions so much as descriptions of of movements of particles—including LD’s own particles—that Shelly will interpret as predictions. This seems promising, but the problem remains that Shelly could look at that entire story in the readout (it should all be there already), and similarly defy it, thus either rendering it wrong and in need of correction, or at least leading to a kind of loop lasting either until Shelly gives up, at which point another human may take over for her. (I won’t consider what happens once two people are involved, but I’m thinking it’s going to make things much tougher for LD.)

The point here is that a paradox arises in the suggestion that LD could predict that its prediction will be wrong. Suppose we think that all the events of the universe happen all at once, time can arbitrarily be thought of as going forwards or backwards, and so on. Shelly can still flip to some arbitrary point near in the future, and decide to do something to change it. And remember, this isn’t about vaguely suggesting some event will happen—LD must know exactly how and when any given event will happen.

Seemingly, Shelly’s awareness of the report is LD’s central source of frustration. Having not seen the readout, she would have done as LD predicted. But maybe Shelly didn’t need to see a readout for her mind—or for any mind at all—to thwart DL (which is a good thing, as it’s occurred to Shelly that she can use LD as a testing grounds for optimizing her future… yikes). I’ve so far intentionally ignored another complications related to the mind: mental causation. I’ll touch on this briefly.

Mental causation, which I’ve written about here, amounts to a difficulty in parsing out mind-body interaction, including how the activity of one can lead to changes in the other. There’s also the question of how thoughts can causally influence one another.

Thoughts can be propositional—”I need to take out the trash” or “Is that noise downstairs an intruder?” or “I’m Ok to drive”—or can simply result from perceiving something, as when the rope in your shed looks for a moment like a big snake.

Mind-body interaction may thwart LD because, as thoughts exert influence on one another and the physical world around around them (whatever that means), it’s not as if neurons are knocking into each other like billions of tiny billiard balls, and it’s certainly not as if thoughts are knocking into neurons or into each other in this way.

This also leads to questions about how the body gives a mind its phenomenology—that is, its experiential content. When one animal has a thought that it attempts to send out into the world by moving its mouth and pushing air through its vocal cords, the elastic matter around the mouth is perturbed in such a way that, should it come in contact with, say, an ear drum, a sound will result in the mind of a hearer; often that sound will carry meaning.

Perhaps this is akin to a “What Mary Didn’t Know” sort of argument. That is, air molecules vibrating at 440Hz may be registered by a computer as the note A, but a conscious observer may attach something further to that sort of event: the mental representation of a pitch.

In other words, when LD is tracking some oscillating bits of matter that enter an ear canal, thus setting off a chain of events leading to neurons firing and so on, LD might run into some difficulty in accounting for what happens when that chain of events culminates in the mental experience of sounds, words, meaning. More broadly, LD’s analysis will need to explicitly account for mental representations such as smell, sounds, anxiety, nostalgia, and so on, and will need to do so in some way that counts as “all-seeing.” What that status could possibly mean in this context gets at the heart of the mind-body problem, as well as the problem of understanding the nature of consciousness in general.


  1. F.W. Truscott and F.L. Emory (translators), A Philosophical Essay on Probabilities, New York: Dover, 1951, page 4.
  2. I’m not claiming that only humans are conscious, nor that only creatures of Earth are conscious. I emphasize this because I’ve seen such discussions derailed by comments like, “But why do you assume only humans are conscious?!? Oh, beware the geocentric hubris of Earth-bound thinkers!”

Dan Jacob Wallace

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