by Lee McIntyre
The problem of demarcation was then taken up by perhaps its greatest champion, Karl Popper. Popper understood—even before Logical Positivism had formally failed—that there were problems with pursuing the verification of scientific statements. The positivists had based science on inductive inference, which undercut the idea that one could prove any empirical statement to be true. David Hume’s famous problem of induction prevented the sort of logical certainty that the positivists coveted for scientific statements.6 Even if they could not be proven, however, weren’t scientific assertions nonetheless uniquely meaningful, given the fact that in principle they could be verified? Popper thought not, and regarded the pursuit of “meaning” as another mistake in the positivists’ approach. What made science special, he felt, was not its meaning but its method. Popper thus set out, in the winter of 1919, to try to solve the problem of demarcation in another way—by renouncing both verification and meaning, focusing instead on what he called the “falsifiability” of scientific theories: the idea that they must be capable of being ruled out by some possible experience.
What concerned Popper was the difference between statements like those in astrology—that seemed compatible with any evidence—and those of science, that take some risk of being wrong. When an astrologer produces a personalized horoscope that says “You are sometimes insecure about your achievements and feel like an imposter” this can feel like a stunning insight into your inner-most thoughts, until you realize that the same horoscope is used for all clients. Contrast this with what happens in science. When a scientist makes a prediction, it comes with an understanding that if her theory is correct you will see what was predicted. And if you do not see that result, then the theory must be flawed.
Popper used this sort of contrast to think about the possible methodological difference between science and nonscience. He was searching for a way to forgo the impossibly high standard which said that scientific statements must always be proven by their evidence, but would still allow evidence to count. And then it hit him. If the Logical Positivists and others were searching for a way to differentiate science from nonscience—but were blocked from being able to say that scientific statements were verifiable because of Hume’s problem of induction—why not instead follow the path of deductive certainty that was already enjoyed by logic?
Those who have studied formal logic know that the simplest and most famous deductively valid inference is modus ponens, which says “If A, then B. And A. Therefore B.” No problem here. No need to check to see whether it “makes a difference to our experience.” Deductive arguments are and always will be valid because the truth of the premises is sufficient to guarantee the truth of the conclusion; if the premises are true, the conclusion will be also. This is to say that the truth of the conclusion cannot contain any information that is not already contained in the premises. Consider the following valid argument:
If someone was born between 1945 and 1991, then they have Strontium-90 in their bones.
Adam was born in 1963.
Therefore, Adam has Strontium-90 in his bones.7
The problem with scientific statements, however, is that they don’t seem to follow this form. For hundreds of years before Popper, they were accepted as being inductive, which meant that the reasoning looked more like “If A, then B. And B. Therefore A.” For example:
If someone was born between 1945 and 1991, then they have Strontium-90 in their bones.
Eve has Strontium-90 in her bones.
Therefore, Eve was born between 1945 and 1991.
Obviously, this kind of argument is not deductively valid. The fact that Eve has Strontium-90 in her bones is no guarantee that she was born between 1945 and 1991. Eve might, for example, have grown up near a nuclear reactor in Pennsylvania in the late 1990s, where it was found that Strontium-90 was present as a result of environmental contamination. Here the form of the argument does not guarantee that if there the premises are true, the conclusion will be true. With inductive arguments, the conclusion contains information that goes beyond what is contained in the premises. This means that we will have to engage in some actual experience to see if the conclusion is true. But isn’t this how we do science? Indeed, when we are engaged in reasoning about empirical matters, we often seek to go beyond our firsthand experiences and draw inferences about those situations that are similar to them. Even though our experience may be limited, we look for patterns within it and hope to be able to extrapolate them outward.
Suppose we are interested in a straightforward empirical issue such as the color of swans. We’ve seen a lot of swans in our life and they have all been white. We may therefore feel justified in making the assertion “All swans are white.” Is this true? We’ve made our observations and have come up with a hypothesis, but now it is time to test it. So we make a prediction that from now on every swan we see will be white. Here is where it gets interesting. Suppose that this prediction turns out to be fulfilled. We may live our whole lives in North America, and, as it turns out, every single swan we ever see is white. Does this prove the truth of our assertion? No. It is still possible that someday if we go to Australia (or just open Google), we will see a black swan.
When we are trying to discover empirical truths about the world, we are hampered by the fact that our experience is always finite. No matter how long we live, we cannot possibly sample all of the swans who have lived or ever will live. So we can never be certain. If we wish to make blanket statements about the world—sometimes instantiated in scientific laws—we face the in principle worry that some future piece of evidence may come along to refute us. This is because the form of the argument that we are using here is inductive, and inductive inferences are not deductively valid. There is just no way to be certain that the rest of the world will conform to our limited experience.
Science nonetheless works pretty well. Although it may not guarantee the truth of our assertions, we are at least gathering evidence that is relevant to the warrant for our beliefs. And shouldn’t this increase the likelihood that our general statements are true?8 But why settle for this? Popper was bothered by the inductive form of inference used by positivists and others as the basis for science. But if that is its logical foundation, how can we possibly demarcate science from nonscience? To admit that “we could be wrong” doesn’t sound like much of a distinction. Popper sought something stronger. He wanted a logical basis for the uniqueness of science.
Popper didn’t have to look far. The inductive argument that we used above has a name—“affirming the consequent”—and it is a well-known fallacy of deductive logic. But there are other, better forms of argument, and one of the most powerful—modus tollens—is deductively valid. It works like this. “If A, then B. And not B. Therefore, not A.”
If someone was born between 1945 and 1991, then they have Strontium-90 in their bones.
Gabriel does not have Strontium-90 in his bones.
Therefore, Gabriel was not born between 1945 and 1991.9
This was Popper’s insight: this, he felt, was the logical basis for scientific inference. Just because science seeks to learn from empirical facts about the world, this does not mean that it is doomed to the problems of inductive inference. For if we look at the argument above, we can see that it is possible to gather empirical evidence and learn from it in a negative way, such that if our test does not work out, we must revise our general assertion. Like the Logical Positivists, Popper was still relying on empirical evidence. But now, instead of that evidence making a difference to our experience so that it could be used for verification, evidence counted because the theory at hand was capable of being refuted by it.
Remember that black swan? If we’d seen one, it would have caused us to revise our hypothesis that “All swans are white.” A single counterexample has the power—through modus tollens—to affect our blanket statements about the world. And that provided a way, Popper thought, for giving up on the idea of verification in science. If we wish to demarcate science from
nonscience, we have to ask a simple question: is the general statement that we have made about the world capable of being refuted by some possible experience, even if we have not had and may never have that experience? If the answer is no, then it is not scientific.
Fortunately for Popper, a real-life example of good science was close at hand. In fact, it may have been what inspired his theory. In May 1919, Arthur Eddington set out on an expedition to take photographs of the stars during a total solar eclipse. This was crucial for the confirmation of Einstein’s theory of general relativity. As Popper explains:
Einstein’s gravitational theory had led to the result that light must be attracted by heavy bodies (such as the sun), precisely as material bodies were attracted. As a consequence it could be calculated that light from a distinct fixed star whose apparent position was close to the sun would reach the earth from such a direction that the star would seem to be slightly shifted away from the sun; or, in other words, that stars close to the sun would look as if they had moved a little away from the sun, and from one another. This is a thing which cannot normally be observed since such stars are rendered invisible in the daytime by the sun’s overwhelming brightness; but during an eclipse it is possible to take photographs of them. If the same constellation is photographed at night one can measure the distances on the two photographs, and check the predicted effect. … Now the impressive thing about this case is the risk involved in a prediction of this kind. If observation shows that the predicted effect is definitely absent, then the theory is simply refuted. The theory is incompatible with certain possible results of observation—in fact with results which everybody before Einstein would have expected.10
In other words, the falsifiability of Einstein’s theory was a prime example of the proper way to do science. In one fell swoop, Popper claimed to have simultaneously solved the problem of demarcation and the problem of induction. That is, since science is not based on induction, it no longer mattered. He had now found a way for empirical observations to make a direct difference in testing our general assertions about the world. And, through modus tollens, this was deductively valid. It is important to understand that Popper was not claiming that his criterion of falsifiability was a way of demarcating meaningful from meaningless statements. Unlike the positivists, Popper did not need to use meaning as a proxy for verifiability, since he had found a direct way to tell the difference between scientific and nonscientific statements.11 It is worth pointing out that falsifiability thus purported to identify not only what was special about science but also what was wrong with those inquiries that were merely pretending to be scientific.
We have already mentioned the example of astrology—which goes back to Popper’s day—but consider here a more contemporary example. In 1981, the state of Arkansas passed Act 590, which required that public school teachers give “balanced treatment” to “creation science” and “evolution science” in the biology classroom. It is clear from the act that religious reasons were not to be offered as support for the truth of creation science, for this would violate federal law. Instead, the curriculum was expected to concentrate only on the “scientific evidence” for creation science. But was there any? And, how precisely was creation science different from creationism?
The act held that the existing situation could not stand, since teaching evolution alone could be taken as a violation of the separation between church and state, to the extent that this would be hostile to “theistic religions” and would tend to favor “Theological Liberalism, Humanism, Nontheistic religions, and Atheism in that those faiths generally include religious belief in evolution.”12 The strategy here was clear: not only were the proponents of creation science attempting to show that it was not religion, they were suggesting that evolution very nearly was religion. But since it was unacceptable to fight this battle in a religious venue, creation science advocates held that they merely wanted an equal chance to offer their views as a scientific contender to Darwin’s theory of evolution by natural selection.13
The fate of this particular piece of legislation—and the lawsuits that followed—will be discussed later in this chapter, and revisited in chapter 8, with intelligent design theory, which took a second swing at trying to get creationism into the public schools. For now the question is a philosophical one: could falsification identify what might be wrong with creation science? Some felt that it could, for just as with the earlier claims of astrology, it seemed that the main thesis of creation science—that God created the universe and all of the species within it—was compatible with any evidence. Didn’t the discovery of 65-million-year-old dinosaur fossils conflict with the 6,000-year timeline in the Bible? Not really, the creation scientists contended, for surely an omnipotent God could have created the entire fossil record! I hope it is clear from our earlier consideration of the problems with astrology that this sort of tendency to explain away any contrary evidence is not a shining example of falsifiability. Whereas true science goes out on a limb to test its theories against experience, creation science refused to change its theory even when there was evidence against it. Add to this the fact that creation science had precious little to offer as positive evidence in its favor, and many were willing to dismiss it as nothing more than pseudoscience.14
The virtues of falsification are clear. If Popper had found a way to solve the problem of demarcation, philosophers and scientists now had a powerful tool for answering the question of what is special about science. They also had a mechanism for dismissing and criticizing those practices—such as astrology and creationism—that they did not want to accept as scientific; if they were not falsifiable, they were not scientific. An added benefit of Popper’s approach was that he had found a way for a theory to be scientific without necessarily having to be true.15 Why did this matter? In seeking a criterion of demarcation, it mattered a great deal to those who were versed in the history of science, and understood that some of the greatest scientific minds of the last few millennia had said things that later turned out to be false. It would be wrong to think that they weren’t scientists. Even though Ptolemy’s geocentric theory was later replaced by Copernicus’s heliocentric one, this did not mean that Ptolemy wasn’t a scientist. He had based his theory on empirical data and had pushed things forward as far as he could. What mattered is that his claims were falsifiable, not that they were later falsified.
It would be easy to imagine that Popper’s new criterion of demarcation was also a vindication of the idea of “scientific method,” but that would be far from true. In fact, Popper was one of the earliest and harshest critics of the idea that there was such a thing as “scientific method.” In his most definitive statement on the subject, appropriately titled “On the Non-Existence of Scientific Method,” Popper wrote “As a rule, I begin my lectures on Scientific Method by telling my students that scientific method does not exist.”16 Elsewhere, he writes:
The belief that science proceeds from observation to theory is still so widely and so firmly believed that my denial of it is often met with incredulity. … But in fact the belief that we can start with pure observations alone, without anything in the nature of a theory, is absurd; as may be illustrated by the story of the man who dedicated his life to natural science, wrote down everything he could observe, and bequeathed his priceless collection of observations to the Royal Society to be used as inductive evidence. This story should show us that though beetles may profitably be collected, observations may not.17
It is important here to remember the distinction between saying that there is a “scientific method” and saying that there is some methodological difference—such as falsifiability—between science and nonscience. Although Popper is unequivocally rejecting the idea of “scientific method,” he still believes that we can have a criterion of demarcation and even one that is methodological in nature.18
This opinion was not shared by some of Popper’s critics, notably by one of his most famous, Thomas Kuhn, who felt that although Popper was correct
to abandon the idea of scientific method,19 one should probably also give up on the idea that there is any distinctive methodological difference between science and nonscience. Note that this does not necessarily mean that one is giving up on the idea that science is “special” or even that there is a way of distinguishing between science and nonscience. Kuhn was not yet ready to do this (though many of his later followers were); instead, he merely pointed out that the process by which scientists actually work has much more to do with nonevidential “subjective” factors in theory choice, such as scope, simplicity, fruitfulness, and the ability to fit a theory with one’s other beliefs, and much less to do with any formal method. And surely this must have an impact on justification.
It is important to understand that Kuhn was not an opponent of science. He was not—although he has been blamed for it—one of those who later claimed that science was an “irrational” process, no better than any other way of knowing, nor did he believe that the social factors that sometimes influenced scientific theory choice undermined its claim to produce credible theories. Instead, Kuhn was at pains to make sure that we understood science for what it really was, feeling that even if we did so it would be no less wonderful. While Kuhn never took it upon himself to try to provide a criterion of demarcation, he did nonetheless feel himself to be a champion of science.20