Saint David

Why does a stone fall when I drop it? The usual answer is: Gravity.

This is a very strange answer, when you think about it. “Gravity” here stands in for the idea that every two masses, if unhindered, will accelerate toward each other. The stone is a mass; the earth is a mass; they accelerate toward each other; so the stone falls toward the earth (and the earth falls upward to the stone, but only a little since it’s so big!).

Where did we get the idea, however, that every two unhindered masses will accelerate toward each other—the idea of gravity? We got it by observing lots of masses, and seeing that, every time we have ever watched them, they have always behaved like this. Every single time we have dropped a rock, it has fallen. For our entire lives, and much longer, the moon has always fallen around the earth, and the earth around the sun. (They don’t fall into each other because they’re already moving so fast in a direction perpendicular to the fall—their fall is a fall from a straight line into a curve.) And every single time we have taken a step, we have fallen—and usually caught ourselves with our advancing foot. Walking is a series of interrupted falls—as you can see when you watch a baby trying it out. (You see it with the baby because the falls often aren’t well interrupted.) Even two billiard balls, in outer space or in another frictionless environment, accelerate toward each other.

Since every time we have carefully observed two masses, we have found that, if unhindered, they have accelerated toward each other, we have decided to invent a law—the law of gravitation—which says that two masses will always accelerate toward each other, if unhindered.

There are two remarkable things about this law.

First, the law comes from the observations we have made. It is not the case that the law exists first—“Masses Always Accelerate Toward Each Other”—and the masses then obey the law, like good citizens. No—rather, the law is an expression of the observed fact that, so far, when we have looked carefully, we have always seen this acceleration.

If I observe a highway full of cars driven by good citizens, I might notice that all of them are driving at or under the 65mph speed limit. This is a case in which a law proclaims what must be done, and the good citizens obey the law. This is not analogous to the situation that surrounds the law of gravity. The situation surrounding the law of gravity is analogous to us noticing that all the citizens are driving at or under 65mph, without any signs or legislation proclaiming the speed limit at all—and then saying, “Hmm, it seems to be a fact that whenever we look, no one goes over 65—let’s hazard a guess about the universal behavior of these citizens, and suggest that they will never go over 65. We’ll call this guess a ‘law.’”

Now that we’ve made our “law,” some innocent might come along and say to us: “Hey, look, these citizens are all driving at or under 65mph. Why are they doing that?”

How should we answer this innocent? We might say, “Oh, yes, well, we’ve just discovered a law. It says that citizens never drive above 65mph. They’re all driving like this because of the law.”

There’s something wrong with this logic. Right? When there were speed limits, the law provided a good explanation for the behavior, because the law was something separate from the behavior; it was what led to the behavior. But in our case the “law” doesn’t add anything to our knowledge of the behavior. It is not separate from the behavior—rather, the “law” is derived from the behavior. The law is nothing but another way of stating that we have consistently observed a set of facts.

When we say that the reason a stone falls whenever we drop it is that it is obeying the law of universal gravitation, we are in a similar position. We invented this law because we always saw unhindered masses accelerating toward each other. There is something very screwy about now turning around and saying that unhindered masses always accelerate toward each other because of the law. Right?

Well, buckle your seatbelts, because it’s about to get even screwier.


The second remarkable thing about our “law” is that it makes a very significant assumption about the ways of the world. It assumes that if unhindered masses have always accelerated toward each other in the past, they always will in the future, too.

This seems, on the face of it, a very reasonable assumption, and I would wager that almost all of us suppose it to be so. Yet in the 1730s a very young philosopher—he was only in his twenties—discovered that it is not a reasonable assumption at all. No one has since been able to set him right.

The philosopher of whom I speak was not at all happy with his discovery. He wrote:

I am first affrighted and confounded with that forlorn solitude, in which I am placed in my philosophy, and fancy myself some strange uncouth monster, who not being able to mingle and unite in society, has been expelled [from] all human commerce, and left utterly abandoned and disconsolate…All the world conspires to oppose and contradict me.

Surely he exaggerates? Surely no truth he could have discovered would be cause for his being “expelled from all human commerce?”

Well, he may have been exaggerating a bit. But imagine that someone were to claim, today, to have uncovered a fatal flaw embedded in all of western science, from Isaac Newton’s to Einstein’s to Stephen Hawking’s and Richard Dawkins’s and beyond. An error in reasoning that renders the entire edifice of western science as good as nothing, by trashing its very foundation: the experimental method itself. And what if this person were not only to claim this, but to be correct in her claim? And what if…no one were to listen to her, to pay her any heed? What if, indeed, she were calumniated, insulted, rejected, and vilified—because she had spoken a truth that no one wanted, or was able, to hear?

Such, indeed, was the fate of David Hume in most contemporary circles. Yet Hume was such a nice guy, such a mild-mannered, good-hearted, claret-loving fellow, that he did find some people ready to forgive him his ideas, and provide him relief from the “most deplorable condition imaginable, environed with the deepest darkness.” The path he took to escape from the awful truth he had discovered—the path he recommended to others as well, if anyone were to read and understand him—was simply to forget about the truth, and allow himself to be distracted:

Most fortunately it happens, that since reason is incapable of dispelling these clouds, nature herself suffices to that purpose, and cures me of this philosophical melancholy and delirium, either by relaxing this bent of mind, or by some avocation, and lively impression of my senses, which obliterate all these chimeras. I dine, I play a game of backgammon, I converse, and am merry with my friends; and when after three or four hours’ amusement, I would return to these speculations, they appear so cold, and strained, and ridiculous, that I cannot find it in my heart to enter into them any farther.

Thank God for backgammon. In another passage Hume elaborates on this idea—rather a strange one for a seeker after truth—writing: “Carelessness and inattention alone can afford us any remedy.”


The godawful truth that Hume discovered is very simple, and very complicated. The reason we suppose that two unhindered masses will accelerate towards each other in the future is simple: because they have always done so in the past. Not because there is a sign out there, or a law passed by God, or by Einstein—there is, as we have seen, no “law” in this sense. Although we have codified our past observations into a mathematical “law” that describes them, and we can fool ourselves into believing that the masses “obey” it, this “law” remains our fiction. The law derives from the behavior, not the behavior from the law. Yet because we have such faith in it, we make a very significant move, and detach it from the past and give it a new status—not as a summary of what we have observed, but as a prediction of what we will observe. It is with this move that a set of facts appears to turn into a law.

But why, Hume asked, do we jump to this conclusion? Why do we assume that unhindered masses will continue to accelerate towards each other in the future as they always have in the past? Is it reasonable to assume this? Why is it that we think, in other words, at this most fundamental level of our physics, that the future will be like the past?

That’s easy, you might say. And it certainly does seem so. The future has always been like the past. Every new morning the sun rises, just as it has for billions of years. Every new experiment with two unhindered masses sees them accelerating toward each other just as they have in every past experiment—and every single drop of a rock, and every single step we take, delivers exactly the falling that we expect. With every new day we experience, again and again, a future in which the same “laws” apply—that is, the same behavior is exhibited—as in the past. And since the future has always been like the past so far, it is surely eminently reasonable to assume that it always will be.

This is the argument that Hume destroyed.

Here it is, in simplest form, applied to our two masses.

1. Two unhindered masses have always accelerated toward each other.
2. Therefore, two unhindered masses always will accelerate toward each other.

Hume saw that these two statements do not an argument make. There is something missing—a hidden premise. In order to make the statements form an actual argument, he added the necessary second premise: the premise without which the argument fails. Here is the argument, then, rewritten:

1. Two unhindered masses have always accelerated toward each other
2. The future will always be like the past.
3. Therefore, two unhindered masses always will accelerate toward each other .

Now the argument definitely works. It is a perfect syllogism. So far so good—but Hume seems to have rescued our reasoning, not demolished it. So what is he so upset about? Well, it’s the second premise. The first premise is unassailable—a simple statement of fact. But how, Hume asked, do we know that the second premise is true? How do we know that the future will be like the past?

In fact, Hume saw, we use exactly the same argument as we did for the masses. In its original, incomplete form, it looks like this:

1. The future has always been like the past.
2. Therefore, the future always will be like the past.

But this argument, as we have seen above with the masses, is invalid. It is missing the second premise. So let’s put the second premise in, just as we did above:

1. The future has always been like the past.
2. The future will always be like the past.
3. Therefore, the future will always be like the past.

Something very strange has happened to this argument! It hardly looks like an argument anymore. It is certainly not a valid one—its conclusion simply repeats its second premise, and its first premise is redundant. Something has gone bonkers here.

It’s not too hard to see what has happened. What we have been doing all along—all our lives—has been extrapolating from past experience to predict the future. After repeatedly seeing two unhindered masses accelerate toward each other, experience suggests that two such masses will always do so. But extrapolating from past experience to predict the future only works if the future is like the past. If the future is different from the past, then our past experience is obviously not going to be a good guide to it.

It is thus futile to extrapolate from past experience to decide whether the future will be like the past. Extrapolating from past experience only works if the future is like the past. But that is exactly the thing that we don’t know, and are trying to find out! This is why the syllogism breaks down.

This is why Hume felt “utterly abandoned and disconsolate…a strange uncouth monster…affrighted and confounded.”


Is there any other kind of reasoning we could use to tackle the question of whether the future will be like the past? Is there no escape from Hume’s conclusion? We look around, and around, and around—and discover, to our dismay, that there is nothing else on offer. What could we possibly go by to answer such a question, besides our experience? We cannot go by the laws of physics, because, as we have seen, they are themselves concise summaries of our experience, rather than externally legislated laws.

This would be bad enough, but it comes with an even more serious consequence. Since we have no reason to suppose that the future will be like the past, we can have no reason to suppose that extrapolating from past experience will ever be valid, for any purpose. Remember, extrapolating from past experience always assumes—in that second premise—that the future will be like the past. Extrapolating from past experience is thus always an unreasonable thing to do, since supposing that the past will be like the future is an unreasonable thing to suppose.

Wait, wait, wait. “Okay,” you might say, “so it’s not set in stone that the future will be like the past. Granted. But at least it’s highly probable. This much is obvious. The future has always been like the past—it has been so a hundred quazillion mabillion times, and never once not been like the past—so even though I cannot prove that it will continue to be so, I can at least bank on the extreme likelihood.”

Well, sorry to rain on your picnic, but no. Even supposing that it is probable that the future will be like the past relies on extrapolating from past experience. If you randomly choose cards from a deck a gazillion times, and find that one tenth of the time you pull a king, you would like to conclude that in the future, if you pull enough cards, a tenth of them will be kings. But this assumes that the future will be like the past. If the future is not like the past, there is no reason for you to think this. Whether we are reasoning about certainties or probabilities, we need that second premise. And we ain’t got it.

You can now see why Hume was so devastated by his conclusions. It’s not only that we can’t be certain that the future will be like the past—something that most of us are probably okay with accepting. After all, maybe maybe maybe there is a God who will, tomorrow at noon, decide to change the physics of his cosmos; or maybe the “laws” of physics are time-bound in ways that scientists have not yet outed.

Hume’s reasoning is destructive of far more than this certainty. It doesn’t only trash our certainty—it trashes what we think is probable as well. Since it is invalid to extrapolate from past experience at all, we can have no idea whether masses will continue to do their thing tomorrow. We can have no idea even of the probability that this will happen.

It’s as if we walk into a casino, and are allowed to bet on black or not black. Only, we don’t know what the game is. We have no idea what the odds are that black will turn up, which may range from 0 (it is impossible) to 100 percent (black is the only possible result). We are not playing roulette; we have, in fact, no idea what we are playing. Should we bet? And how? But it is even worse than this. When we walk into a casino, we know what kind of institution we are in, and can assume that it is possible to win if we get lucky. But now we are not even in a casino. We don’t know where we are. We don’t even know what putting money on black means.


Hume was well aware that we all harbor an ineradicable inner belief that the future will continue to be like the past. And not only do we harbor this belief—animals appear to as well. When a cow has once—or at the most twice—touched an electric fence, she shies away from it in the future. She assumes that the future will be like the past, and that the fence will sting the next time as well.

The above sentence may ring funny, because actually we have no idea whether cows can be said to “believe” or “assume” things. Better to say, then, that the cow, once stung by the fence, is afraid of it. The cow associates the fence with pain, and so avoids it. No assumption or belief is required—only an association.

And perhaps, in this, we are not so different from the cow. The problem may not be that of employing human terms for the thinking of the cow, but in not applying bovine terms for the thinking of humans. When we see two unhindered masses accelerate toward each other, over and over, we associate unhindered masses with acceleration. The next time we drop a rock, this association springs up, and we expect it to fall. We have not reasoned about the matter at all. We have not assumed anything, nor do we need to believe. We simply associate, and expect. A two-year old human also expects a dropped rock to fall, without any considerations of logic, without any explicit “assumption” that the future will be like the past. Hume called this association and the expectation it induces “custom”—and believed we shared it with the animals. It is custom, he decided, that really accounts for our beliefs—not reason. Reason is a foil, a scarecrow, a fiction.


The implications of these insights for an evaluation of western science are far-reaching. Again and again, defenders of science against religion, spiritualism, animism, and all kinds of alternative beliefs emphasize that science is “rational”—that is, based on reason—and this is its great calling card. Science is based on reason, and not on feelings, traditions, special knowledge, revelation, or other privileged insights into how things are.

This is, to put it bluntly, a lie. Science is based on extrapolating from past experience, a procedure that Hume demonstrated almost three hundred years ago to be unreasonable, irrational, nothing more nor less than “custom.”

Moreover, this failing in the credentials of extrapolation from past experience does not make the “laws” of science merely uncertain—a concession that every decent scientist will make. No. Rather, it makes the “laws” of science improbable to the point of vanishing. There is no reason to suppose that they will—even probably—have any validity as descriptions of how things are in the future.


In fact, all of us have ample experience of worlds in which the “laws” of science fail to work, in which the future is not like the past. Most of these experiences take place at night, in dreams, though some occur during waking hallucinations and other experiences of altered consciousness. As western science has gained currency over the last several centuries—in large part due to its claim to be so eminently “rational”—the conventional evaluation of dreams and visions has come to see them as quaint and meaningless, arguably useful for cluing a therapist into our concerns, but in themselves of little value. Once upon a time, things were different. Shamans were honored because of the trances they entered; ordinary people went on vision quests; altered states of consciousness were looked on as revelations rather than as delusions. The regularities in the physical world so honored by science were paid less attention than the irregularities of dreams, visions, and other unpredictable phenomena.

We can ask—and I think we should ask—whether our conviction of the “rationality” of western science has not seduced us into excessively honoring the scientific way of thought, and thereby clipping the wings of our own experience. Has this honor paid to “rational” science induced us to shift our attention away from the dreamlike elements of our lives, the irregularities, the many, many instances in which the future is not, in fact, like the past? Do we undervalue these elements, see them as mere add-ons (and subjective, deluded add-ons at that, funny creatures, illusions, chimeras, unreal, insignificant) to the regular, “law-bound” elements supposedly certified by a bastion of rationality that is in fact far from rational? Such an undervaluation would be tragic, and perhaps not only for us but, due to the mechanistic perspectives it promotes, also for our environment and fellow creatures.

There may be better remedies to Humean despair than backgammon. I would even dare to suggest that, in our age, Hume’s insights might be cause not for despair, but for delight.

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