Dan Herbatschek

Søren Kierkegaard was a singular figure. Alongside Nietzsche, he stands as one of the two 19th-century roots of what would later grow into 20th-century existentialism. The two could not be more different—Kierkegaard deeply religious, Nietzsche fiercely antireligious—yet both were brilliant stylists and solitary minds who frame the movement like opposite bookends. Still, Kierkegaard is even harder to place than Nietzsche, for his thought seems to emerge without real forerunners. His philosophy developed less from inspiration than from opposition, shaped by what he rejected and sought to escape. At the heart of his work lies a single pressing question: what does it mean to be a Christian? In what follows, we’ll explore the intellectual climate he resisted, the central ideas he advanced—especially those in his seminal work Fear and Trembling—and finally, the influence he left behind.

Much like Rousseau, Kierkegaard was a critic of modern life, though he wrote a century later, in an age increasingly shaped by cosmopolitanism, science, and industrial growth. Rousseau had argued that cultural progress did not amount to moral progress; Kierkegaard, in parallel fashion, insisted that it also did not signify spiritual or religious progress. His opposition was not directed at secularism—still not fully explicit in his time, since it makes more sense to date the rise of secular modernity to the later 19th century rather than the mid-19th century when Kierkegaard was writing—but at a Christianity reshaped to fit the demands of Reason. Above all, this rationalized version of the faith was represented by Hegel. Kierkegaard’s critique was aimed not at atheists or nonbelievers, but at Christians he thought had profoundly misunderstood their own religion. His message to a still largely Christian Europe was simple and radical: it did not truly grasp what Christianity meant.

Kierkegaard was also the most intricate philosophical writer since Plato—not because his prose was opaque like Kant’s or Hegel’s, but because he deliberately concealed his own voice behind pseudonyms. Like Plato, who presented arguments through his characters, Kierkegaard rarely spoke directly. Almost all of his works were written in the names of imagined figures, each embodying a perspective, and this strategy gave his writings an ironic and aesthetic character.

Born the son of a deeply religious father, Kierkegaard wrestled throughout his life with what it meant to be a Christian. Financially, he lived off an inheritance that sustained him until his death. Socially, he was isolated—recognized in Denmark, yet also mocked. Anecdotes (perhaps exaggerated) tell of children in Copenhagen chasing him through the streets, taunting him with chants of one of his book titles: “Either/Or, Either/Or!”

The defining moment of his personal life came at age 28, when he ended his engagement to Regine Olsen after three years. He concluded that marriage was incompatible with his calling. To Kierkegaard, his mission of uncovering Christianity’s true meaning demanded a particular way of life, and the obligations of marriage would have undermined that vocation.

Kierkegaard had a deep disdain for Hegel; he is perhaps the clearest representative of the mid-19th-century backlash against Hegelian philosophy. At one point, Kierkegaard remarked that if Hegel had written his Logic—one of his key works—and then admitted in the preface that it was merely a thought experiment, he would have been “the greatest thinker who ever lived.” But since Hegel presented it as truth, Kierkegaard concluded, he was “merely comic.” That remains one of the sharpest barbs ever delivered by one philosopher at another.

What infuriated Kierkegaard was Hegel’s drive to systematically subsume every dimension of human life—what Hegel himself called the “shapes of spirit”—into one unified system. For Hegel, nothing was beyond synthesis; every experience and aspect of reality could be reconciled into a comprehensive whole. Kierkegaard, by contrast, saw life as fractured. Human existence consists of distinct modes of living that cannot be harmonized. The individual must therefore make genuine choices—irreducible decisions between incompatible ways of life. This insistence on the necessity of choice is a cornerstone of Kierkegaard’s existentialist legacy.

He outlined three broad categories of existence. The first is the aesthetic life. This way of living revolves around pleasure—or more precisely, around whatever is interesting. It is fundamentally self-centered, though not necessarily immoral. Rather, it is amoral: it pursues its aims without concern for morality. The purest illustration of this comes in his literary masterpiece, Diary of a Seducer. There, Kierkegaard sketches a man who meticulously plots his entrance into a woman’s life—first arranging to be seen on the edges of her vision, then engineering a chance encounter, then ensuring a friend casually mentions his name. Gradually he insinuates himself into her world, causes her to fall in love with him, and then abandons her forever. The seducer’s goal is not cruelty but novelty—he acts out of a relentless pursuit of what is “interesting.”

The second category is the ethical life. This is the ordinary, social way of living, governed by reason and shared norms. In this mode, we justify our actions in terms that others can recognize, and we aim for harmony with those around us. Most people live primarily within this ethical framework. That doesn’t mean we never fail, but our lives are structured by moral rules and mutual understanding.

Within this stage, Kierkegaard describes an extreme figure: the knight of infinite resignation. Such a person is capable of surrendering every personal hope or desire, giving up all aesthetic satisfactions, for the sake of a moral duty. Not everyone can reach this level of ethical consciousness, but for Kierkegaard it represents its highest expression.

The third and highest stage of existence, for Kierkegaard, is the religious life. Unlike Hegel’s system, this third stage is not a reconciliation or synthesis of the first two. That alone signals how far Kierkegaard stands from Hegel—not just in his mocking comments, but in the very architecture of his thought. Hegel’s triads always resolved in a synthesis that integrated opposites; Kierkegaard’s triad resists that logic. The religious does not mediate between the aesthetic and the ethical. Instead, it stands apart, more akin to the aesthetic in its individualism, but infinitely deeper: it is the solitary, unmediated relation between the individual and God.

To recap: the aesthetic is the pursuit of pleasure, novelty, and the interesting; it may be refined but remains self-focused. The ethical is social and rational, embedded in human community and shared norms. But the religious transcends both: it is neither social like the ethical nor sensual like the aesthetic. Its essence lies in the individual’s absolute relation to God.

This becomes Kierkegaard’s central mission—to grasp the nature of this third stage, the religious life. His claim is that it has never truly been understood, and that Hegel’s attempt to fold religion into reason and society only obscured it further. Kierkegaard even confesses, in one of his writings, that while his age prides itself on sophistication—everyone reading Hegel’s Logic and Encyclopedia, marveling at scientific discoveries, inventing new machines for convenience—he remains stuck on the fundamentals. “I do not even understand Abraham and his faith,” he admits. That, he insists, is the mystery he wants to understand.

His pseudonymous voices acknowledge their own aesthetic tendencies, since Kierkegaard believes the religious cannot properly speak for itself. By its nature, it resists direct explanation. And the ethical perspective, he argues, is constitutionally incapable of grasping the religious. These are startling claims. We are used to thinking of religion and ethics as intertwined; Kierkegaard flatly denies this. To interpret religion ethically is to misinterpret it. The two stand worlds apart. For Kierkegaard, the biblical God demands acts that overturn ordinary moral reasoning. Even the central Christian command—forgiveness—is, in human terms, “unethical.”

Let’s pause to ask why Kierkegaard would go so far as to call forgiveness “unethical.” At first glance, this seems outrageous—forgiveness is celebrated in Christianity and many other religions as the pinnacle of moral life. Yet Kierkegaard insists otherwise. Ethics, he argues, rests on the rules of justice: wrong actions demand punishment, and those rules are binding. Forgiveness, however, interrupts this structure. It cancels justice’s demand by telling the guilty: even though you deserve punishment, you will not receive it. In Kierkegaard’s terms, forgiveness is the religious breaking into the ethical domain and overturning one of its fundamental principles.

And it gets more radical still. For Kierkegaard, religious faith cannot be reconciled with Reason, contrary to the claims of Hegel and the medieval Scholastic tradition. Recall that Thomas Aquinas in the 13th century had famously attempted to unite Christianity with Aristotle, arguing that Reason and faith complemented each other. Aquinas believed that rational inquiry into creation—the order and beauty of the natural world—was itself a form of worship, a way of moving closer to God. Across much of Western history, especially during its more optimistic periods, faith and Reason were viewed as harmonious, two powers of the human spirit working in concert.

Kierkegaard is uncompromising in rejecting this. Faith, he insists, is not simply beyond reason (arational)—it is actually against reason. It is irrational. This is a bold and unsettling claim.

Another hallmark of Kierkegaard’s thought is his insistence that “truth is subjectivity.” For him, truth is not primarily about objective reality but about the lived, inward experience of the individual subject. He makes this point forcefully in Concluding Unscientific Postscript. What matters in philosophy, he says, are not abstract descriptions of the world but the questions that compel a person to confront how he or she will live. Scientific, objective knowledge is thereby demoted to second-class status. Being human, Kierkegaard argues, cannot be understood through the methods of science. The inner life is fundamentally different from the outer world.

This emphasis on subjectivity was one reason Kierkegaard became so important for later thinkers, especially the early phenomenologists such as Martin Heidegger. For them, as for Kierkegaard, the ultimate philosophical question is about a way of life—and a way of life is not a purely intellectual matter but a passionate one. Choosing between the aesthetic, ethical, and religious modes of existence is not a decision that science or reason can settle. It is, instead, a matter of human existence itself: I must decide, I must commit, I must “bet” on one path of life, even without certainty.

Let’s turn now to Kierkegaard’s most famous illustration of such a choice, drawn from his masterpiece Fear and Trembling. In that book, he takes the biblical Abraham as the model of what he calls the knight of faith. You’ll recall the story: Isaac, Abraham’s beloved and long-awaited son, born in his old age, is demanded of him by God. Abraham is commanded to sacrifice Isaac, and he sets out to obey. Kierkegaard’s pressing question—characteristic of his approach in this strikingly beautiful text—is: what is happening within Abraham as he climbs the mountain, knife in hand, prepared to kill his only son? What is Abraham saying to himself to make sense of his action? That is what Kierkegaard longs to grasp. And here we see again how he differs from other philosophers—while they concern themselves with universal principles, the progress of science, or abstract metaphysics, Kierkegaard lingers in the Old Testament, struggling to comprehend Abraham.

First, he rules out a common interpretation: Abraham does not treat the ordeal as a divine prank or a mere test, as though he were secretly confident that God would not require him to go through with it. Second, Abraham’s stance is not one of tragic resignation either. If it were, he would be what Kierkegaard calls a knight of infinite resignation. That attitude would be fully rational, even philosophical, and comparable to the Buddha’s claim that all life is suffering. Abraham might have said: “I was wrong to love my son so deeply. Everything is fleeting. I will obey God’s command, and though it breaks me, I resign myself.” Such a view is intelligible—it accepts the loss and suppresses attachment. In Kierkegaard’s framework, that would count as an ethical response, a surrender of personal desire in obedience to a higher necessity.

But Kierkegaard insists that Abraham’s position is different, stranger, and more paradoxical. Abraham must believe both that he will kill Isaac and that he will somehow still keep Isaac—that Isaac will live. He walks up the mountain convinced that he will plunge the knife into his son, and simultaneously convinced that he will not lose him. To our ears this is contradictory, even absurd—and Kierkegaard agrees. This is precisely what makes it faith. Faith, he says, is not compatible with Reason; it is literally irrational. The Latin phrase credo quia absurdum—“I believe because it is absurd”—though wrongly attributed to Tertullian, captures Kierkegaard’s view more closely than that of any other modern thinker. Faith is the commitment to something that defies reason.

With this in mind, we can better see why faith stands apart from ethics. God’s command to Abraham—to kill his innocent son—is, by ordinary ethical standards, monstrously wrong. Just as forgiveness subverts the demands of justice, God’s command lifts Abraham out of the moral order altogether. Religion, for Kierkegaard, intrudes into the ethical sphere only to suspend and overturn it. And because faith is essentially a solitary relation between the individual and God, it cannot be mediated, justified, or even communicated to others. It is incommensurable with the social world, with rationality, and with ethics itself.

Kierkegaard is making a sharp point here, one that feels surprisingly modern. I don’t mean his insistence on irrationalism, but rather his observation that rationality is tied to the social. To act rationally is to be able to give reasons that others can recognize, to justify oneself in a way that makes sense to a community. Rationality, in this sense, requires communication.

Faith, by contrast, is none of these things. It is not rational, not social, and it cannot be communicated. Because of this, a knight of faith cannot be identified by any outward behavior. Take Abraham: he could never explain his action to another person. Nobody would understand him, because what he was doing was irrational by every human standard. Likewise, you could not point to any external sign that marks someone as a knight of faith. Kierkegaard has a marvelous passage where he says that the knight of faith might look no different from an ordinary tax collector. He wakes up in the morning, washes his face and hands, skims the newspaper, drinks his coffee, and goes to work. To outside observers searching for some trace of transcendence in his behavior, there is nothing to find. The inward reality of faith has no outward signature.

Kierkegaard’s sharp separation of the aesthetic, the ethical, and the religious—as incommensurable forms of life—creates real difficulties in understanding him. It also explains his curious literary strategy. His use of pseudonyms was not accidental. Because these spheres of existence cannot be reconciled, he needed different voices to explore them. In Fear and Trembling, for instance, the narrator is explicitly aesthetic. He admits openly: “I am an aesthetic character; I find Abraham’s faith fascinating and puzzling. I cannot grasp it, nor do I see how anyone could.” From Kierkegaard’s standpoint, only a true knight of faith could understand another knight of faith, and such figures are vanishingly rare. For the rest of us, we can only view them from the outside—through the aesthetic lens, or the ethical one. Both are distortions, but Kierkegaard suggests that the aesthetic point of view, strangely enough, gets us closer to glimpsing the religious than the ethical does.

This raises a further problem: if these forms of life are incommensurable, how can one ever move from one to another? How does someone transition from the aesthetic to the ethical, or more radically, from the aesthetic to the religious? Kierkegaard insists that there is no gradual ladder, no dialectical progression, no educational path that carries us from one to the next. Unlike Hegel, who always resolved opposites into a synthesis, Kierkegaard divides existence into starkly separate possibilities. The religious does not absorb or reconcile the other two; it is wholly other.

That is why Kierkegaard titled one of his most famous works Either/Or. As the (perhaps apocryphal) story goes, children in Copenhagen would tease him by shouting this title as he passed. The phrase captures his conviction: when it comes to ultimate ways of life, you must simply choose. And this choice is groundless, without reasons. Why? Because any reasons you might give are drawn from within the form of life you already inhabit. If you are living ethically, the ethical mode can only offer reasons to remain ethical, not reasons to leave it. The gulf between forms of existence is unbridgeable, and choice comes down to a leap.

Does this logical impasse trouble Kierkegaard? Not at all. His reply is consistent: of course it makes no sense—it is irrational. And that is precisely the point.

To conclude: Kierkegaard occupies a singular place among modern thinkers. Many philosophers—especially those tied to 20th-century existentialism—flirted with irrationalism. William Barrett even titled his classic overview of existentialism Irrational Man, pointing to how central the so-called irrational became for this tradition. Yet Kierkegaard stands apart. He is the most sophisticated and relentless critic of rationality in modern philosophy, and at the same time one of the most passionate, emotionally charged writers the field has ever produced.

What, then, is his legacy? In the 20th century, Kierkegaard left two distinct marks. On one side, he deeply influenced liberal Protestant theology, inspiring a movement within religious thought. On the other, though he was too unusual to shape mainstream philosophy widely, his existential view of religion had a surprising impact on the century’s leading atheist existentialist, Martin Heidegger. Heidegger leaned heavily on Kierkegaard’s insights in his landmark book Being and Time.

So what can we finally say about Søren Kierkegaard? Above all, he achieved the aim he set for himself. He crafted a philosophy that was personal to its core, a philosophy centered on subjective truth—truth bound up with the most profound and agonizing choices of human existence. Rejecting the standards of conventional rationalism, Kierkegaard turned philosophy back toward the life of the individual: a single finite life, filled with decisive moments that shape its meaning. If one were to ask Kierkegaard, “What should I do?” his counsel, in the end, would likely be distilled to a single imperative: choose.

Recognizing that it’s no easy task to pin down the essential features of science, let’s return to the story at hand. In Lecture Two we saw how a piece of Einstein’s general theory of relativity became a striking and inspiring case study of successful science for figures like Popper. This time, we rewind further, to Einstein’s special theory of relativity, first published in 1905 when Einstein was only 26.

This was a revolutionary advance in physics, though here we’re more interested in the philosophical impact—which is just as significant. The term “special” doesn’t mean “especially brilliant,” but rather “a special case of relativity.” Special relativity applies to constant-velocity motion, where objects move without forces that accelerate or redirect them. I’ll occasionally call this “inertial motion.” The general theory of relativity, by contrast, applies to all motion—accelerated as well as uniform—and so in a sense is the “grander” of the two. For our purposes, though, sticking with the simpler special theory will do.

An important historical note: many of the concepts most often attributed to Einstein weren’t originally his. His genius lay less in inventing the raw pieces of the theory than in weaving them together into a consistent framework, which was the truly difficult part.

We already know that inertial motion—motion at constant speed in a straight line—can only be described relative to some chosen frame of reference. Galileo argued this in the early 1600s: a ball rolls the same way on land as it does on a ship gliding steadily across calm waters. Clearly this was a thought experiment, since no ship actually sails in a perfectly uniform way, but his insight holds. You can sense when a ship speeds up or turns; but with the windows closed, there’s no experiment you can run that tells you whether you’re stationary or coasting smoothly forward. This idea came to be called the “principle of Galilean relativity.”

Since mechanical laws don’t distinguish between one state of steady motion and another, we might as well treat any inertial state as if it were at rest. In this sense, Einstein didn’t invent relativity—he extended and generalized Galileo’s principle.

Put differently: suppose you and I drift past one another in deep space. Neither of us can determine who is “truly” moving. All we can say is how each of us moves relative to the other. The natural temptation is to conclude that the question “which one is really in motion?” has no real meaning—at least no empirical meaning, since no test could answer it. But the situation is a little trickier than that. Other parts of physics—notably electromagnetism and light—seemed to suggest there might be some way to define “absolute” motion.

Here’s why: James Clerk Maxwell, the 19th-century Scottish physicist, had shown that light behaves like an electromagnetic wave. At the time, waves were assumed to require a medium—like water for ocean waves or air for sound. The medium for light was thought to be the “aether,” through which light rippled as sound ripples through air.

Think of sound in water. If you’re still with respect to the water, you’ll record the same speed of sound in every direction. But if you swim through the water, the sound’s measured speed will vary: it will seem faster if you’re heading toward the wave, slower if you’re moving away. The same logic applies to catching or fleeing from a thrown baseball: relative to you, its speed depends on your own motion.

Notice the trick—we’re talking about how things appear to you as the moving observer, not about the “true” underlying motion.

By the same reasoning, light should act this way too. If the Earth is plowing through the aether at a steady pace, then measurements of light’s speed should vary depending on the direction. In one direction, Earth would be “chasing” the light, so the light should seem slower; in the opposite direction, Earth would be moving away, so the light should seem faster.

Now, it wasn’t obvious that moving relative to the aether was identical to moving relative to space itself, but since the aether was thought to permeate everything, it looked like a natural candidate for an absolute frame of reference.

Here’s the kicker: experimenters repeatedly tried to detect Earth’s motion through the aether, but no matter what point in Earth’s orbit they measured from, they always got the same answer—the same speed of light. This was baffling. It was as if a baseball always approached you at the same speed whether you ran toward it or away from it.

Einstein didn’t discover this puzzling constancy of light speed—it had already been observed in experiments and predicted in Maxwell’s theory decades earlier. What Einstein did was far more daring: he accepted the experimental results at face value and sought a way to reconcile them with relativity.

This set up a genuine puzzle: on the one hand, relativity tells us that all steady motion is relative; on the other, light insists on traveling at a fixed speed no matter who’s moving. At first glance, those two principles flatly contradict each other.

Let’s spell it out. Galilean relativity says that to speak of motion you must reference some frame—“I’m going 55 miles per hour relative to the highway.” But light, in a vacuum, is said to move at about 670 million miles per hour, and that’s not relative to anything. Even if you were to chase after a beam of light at tremendous speed, the light would still recede from you at exactly 670 million miles per hour.

So it looked as though one of the principles had to be false. Nearly everyone assumed the culprit was the claim that light’s speed is constant regardless of the observer. But those stubborn experimental results wouldn’t vanish.

Scientists, unwilling to abandon the older framework, resorted to some desperate patches. One suggestion was that Earth dragged along a bubble of aether around it, meaning that locally we were always at rest with respect to the medium. Another, even more radical, was that instruments themselves were altered by motion: measuring rods shrank and clocks slowed whenever Earth moved through the aether.

That sounds bizarre, though it wasn’t entirely implausible within 19th-century physics. If electromagnetic forces held matter together, and those forces propagated through the aether, then perhaps motion through the medium distorted objects. Still, this “explanation” was a stretch: it said the speed of light really did vary, but conveniently, all our measuring tools were distorted in just the right way to hide it from us.

Einstein’s leap was to stop fighting the evidence. He accepted the data as it stood. Instead of assuming the contradiction was real, he asked whether the contradiction only appeared because we were holding on to unexamined assumptions about space and time.

The first step was to broaden the principle of relativity. Galileo had said that no mechanical experiment could reveal whether you were at rest or moving steadily. Einstein extended that: no experiment of any kind—mechanical, electromagnetic, or otherwise—can distinguish one uniform motion from another. And light, like Newton’s laws, obeys this principle.

But this reconciliation came with radical consequences. Different observers, moving relative to one another, would inevitably disagree about fundamental things: whether one event happened before another, or how long an event lasted.

To dramatize this, let’s borrow a thought experiment from physicist Brian Greene. Imagine two presidents, Enginefacer and Caboosefacer, about to sign a treaty. To ensure fairness, they sit equidistant from a light bulb in the middle of a train car, which is rolling at constant velocity across the border between their nations. When the car’s midpoint passes the border, the bulb flashes. Each president, upon seeing the flash, immediately signs. From their perspective inside the train, both sign simultaneously.

But observers standing on the platform see something different. To them, Enginefacer is moving toward the flash while Caboosefacer is moving away. Since light travels at the same speed in both directions, the beam reaches Enginefacer first. Thus, from the platform’s frame, Enginefacer signs earlier.

So which is right? The train observers or the platform observers? The astonishing answer is: both. Each perspective is correct within its own frame. The question “which signing came first?” has no answer until a frame of reference is specified.

This leads to equally strange results for time. If you and I are gliding past each other, my measurements will tell me that your clock runs slow, while you will say the same about mine. It feels impossible—surely both clocks can’t be slower than the other—but that impossibility rests on the pre-relativistic assumption that there’s a single, absolute timeline. Einstein showed that assumption was the mistake.

The same logic applies to space. To me, your meter stick looks contracted; to you, mine does. In our own frames, though, each appears normal.

Let’s look at another well-known illustration. Imagine we measure a barn and a pole at rest, side by side, and confirm they’re exactly the same length. Now picture someone sprinting at seven-tenths the speed of light, carrying the pole through the barn. From our vantage point at the barn, the pole appears shortened, so it seems both barn doors could close with the pole inside.

But from the runner’s perspective, things are reversed: the barn is what has contracted, not the pole, so the barn is too short to ever contain it.

So what actually happens? Both, depending on the frame. We see the doors close around the runner; the runner sees the back door still open and the pole sticking out before the front door shuts. At first this looks contradictory, but the tension vanishes if you calculate when light signals, traveling at their maximum speed, reach each observer from the critical points (the ends of the pole and the barn doors). Once you factor in the travel time of those signals, the apparent paradox dissolves.

Of course, nobody has ever run through a barn at relativistic speeds. But experiments with subatomic particles confirm the strange consequences of special relativity. Muons, for instance, normally decay very quickly. Yet when accelerated close to the speed of light, they “live” about ten times longer, because from our perspective, their clocks slow dramatically.

For us, the exact physics isn’t the main concern. What matters is the conceptual shock: physicists had assumed they knew what they meant by words like “simultaneous” or “length.” Einstein forced a reconsideration: those concepts only make sense when tied to the procedures of measurement and observation.

That realization profoundly shaped both science and philosophy. For decades, much of the debate revolved around how to connect scientific concepts securely to experience.

One of the first to push this idea was not a professional philosopher but a Nobel-winning physicist, P. W. Bridgman. Drawing inspiration from special relativity, he argued that the lesson was conceptual: never again should abstract ideas blind us to what experiments reveal.

Scientists had overlooked the constancy of light’s speed because their theories got in the way. Einstein succeeded by working through the implications of the data instead of forcing the data to match old assumptions. Bridgman’s response was a new view called operationalism.

According to operationalism, every scientific concept must be defined in terms of the operations used to measure or detect it. Take “length”: instead of defining it philosophically as “the amount of space something occupies,” Bridgman says it just is the procedure of laying a meter stick along an object and recording the number. If the object is longer than the stick, you move the stick and keep counting. The operation itself provides the meaning.

His idea was that if we had insisted on defining simultaneity operationally, we would never have been surprised by relativity. Since no signal can travel faster than light, observers moving relative to one another cannot agree on whether distant events happened at the same time. If we had tied the concept directly to measurement, the relativity of simultaneity would have been obvious instead of shocking.

Let’s see how operationalism works when applied more broadly. Strictly interpreted, each measurement procedure defines a distinct concept. For example, “temperature as read by an alcohol thermometer” is technically different from “temperature as read by a mercury thermometer.” If we swap instruments, we’ve switched the operation—and therefore the meaning.

At first glance that sounds manageable, but problems appear quickly. How finely do we draw the line between operations? Does “testing acidity with blue litmus paper while wearing a lab coat” count as the same operation as “testing acidity with blue litmus paper while wearing overalls”? They differ in one respect, however trivial. Bridgman might say in principle we should count these as separate, though in practice we don’t bother. But even if we ignore such trivialities, the challenge remains: how exactly should operations be individuated?

Another issue is that operational definitions can’t regress forever. Eventually, they must bottom out in basic, unanalyzed acts—like “look at the scale and write down the number.” At some stage, scientists must assume certain observations are immediate and unproblematic. Bridgman accepted this, but it sets him apart from philosophers like Karl Popper, who denied that there is a privileged layer of “pure” observation immune from further critique.

Even with these difficulties, operationalism proved highly influential in the early 20th century. In psychology especially, it shaped practice: diagnosing “depression,” for some schools of thought, simply meant scoring below a threshold on a particular test. No extra meaning was allowed beyond the operational definition. The appeal was clear—psychology is complicated, so tying concepts to standardized procedures ensured consistency.

But most philosophers now view operationalism as too restrictive. If every concept must be fully defined by a single measuring operation, science either loses important meanings or cheats by smuggling them back in. Bridgman’s view risked crippling inquiry.

Take “weight” as measured with a pan balance. If the pans balance, the objects weigh the same; if one side falls, that side is heavier. Straightforward enough—except this presumes no hidden forces are interfering. But how do we operationally guarantee that? We could add steps: check for someone pressing on the scale, test for magnetic fields, and so on. Yet the list of possible disturbances is endless. Clearly, our notion of weight isn’t reducible to just the pan balance’s behavior—we assume the balance is tracking something real, not defining it.

The same holds for thermometers. We assume that alcohol and mercury thermometers measure the same property. We even talk about temperatures too small for current instruments to detect. That confidence shows we treat “temperature” as something out there, not something created by the act of measurement.

Here’s another example: a thermometer plunged into the sun would explode, killing the observer. Yet we still believe the sun has a temperature, and we seek instruments capable of measuring it. That assumes an independent concept of temperature—one not reducible to current operational methods.

And this, ironically, is what operationalism was meant to eliminate. Einstein’s revolution had warned against sneaking in extra meaning beyond measurement. Bridgman took that warning seriously, but his solution proved too rigid.

Still, his operationalist call made an important contribution: it reminded scientists to anchor their terms in measurable practice rather than abstract speculation. As we’ll see later, this idea has a long philosophical history, and thinkers had been grappling with these issues for centuries. What Einstein did was make them freshly urgent.