The Pessimistic Meta-Induction

Bryan W. Roberts
Philosophy, Logic and Scientific Method
Ph103 - Lecture 6

Background Reading:
Psillos, Resisting the Pessimistic Meta-Induction,
Chang, Preservative Realism and its Discontents

The relevance of history of science

Last week, we discussed whether or not our best scientific theories might be approximately true, even about the aspects of the world that they describe that are unobservable.

Suppose that science were a job applicant. How would you decide whether or not to give them the job? What would you look at? You would look at their education, surely, but why — and what else?

If you're sensible, you'll look at the applicant's past experience to see how they did. If an airplane pilot crashed every airplane and parachuted to safety, then you probably wouldn't want to hire that pilot. Past behavior can tell you a great deal about current and future behavior.

So, history matters. If we want to get to the bottom of whether our best existing scientific theories are true, we had better look into some of our past scientific theories to compare!

Unfortunately, this exercise apparently does not to go well for the realist.

Historically false theories

The crystalline spheres theory of astronomy

From ancient Greek time until about the 16th century, the celestial objects visible from the sky were generally thought to inhabit concentric spheres around Earth.

Those spheres slowly rotated at different rates around the earth. The first sphere contained the moon, the fourth contained the sun, and the final sphere containing a "fixed" star field.

These spheres were considered "crystalline" as part of a fundamental separation between "heavens" and "earth" that was thought to exist. According to this separation the earth was fundamentally imperfect, whereas the heavens were fundamentally perfect or "crystalline."

The humoral theory of medicine

Greek and Roman medical theory, encoded by the Roman physician Galen of Pergamon, held that maintaining the body's health was a matter of balancing for different fundamental fluids called humors.

For example, the common cold was caused by too much phlegm, which has a cold and moist quality. One could alleviate the condition by trying to raise one's temperature with hot tea, and by expelling the phlegm as much as possible.

If one had received a cut, the resulting blood was considered to be a "hot" condition, which needed to be cooled. The advice was to pour something cold on it to bring down the temperature.

But you don't want to cool it down to fast with something like water. So, patients were encouraged to cool it down more slowly by pouring alcohol on a wound. The technique proved remarkable successful!

Of course, today this theory is known to be almost entirely incorrect, although it is often still visible in folk medicine. We have learned instead that infectious disease is caused by microorganisms such as viruses and bacteria that are transmitted by touch, consumption, and respiration.

The phlogiston theory of chemistry/combustion

Some things are flammable, and some things are not. The difference, it was thought until the mid-1700's, was that some objects contained more of the flammable element phlogiston, which was just like any other element on the periodic table, except that it caused combustion.

Burning was considered to be a chemical reaction that depleted phlogiston, just as a machine depletes fuel. This explained why burned objects appear to have less mass than they did before they were burned. Some of the matter was literally thought to have been expelled.

However, this theory had trouble explaining a number of important phenomena, like the nature of magnesium. Magnesium is just a metal:

But for this reason, it was thought to have no phlogiston in it, and thus should not be flammable. As it turns out, magnesium does burn at hot enough temperatures. But even worse, it gains mass after burning.

This seemed impossible to explain if burning was just depletion of phlogiston. Today, we understand this happens is because burning is a chemical reaction that involves oxygen, which bonds with magnesium to form a new molecule, magnesium oxide. The magnesium thus appears to gain mass because it has bonded with oxygen.

The electromagnetic theory of aether

Our best theory of electricity and magnetism was developed on the idea that light is an electromagnetic wave. How did people think about this kind of wave?

First, think about a water wave. Water is the substance through which water-waves are transmitted. These waves are just the "up-down" motion of individual particles of water.

Water-waves only make sense when there is water, which provides the medium for the wave to propagate through.

So, once it was discovered that light behaves like a wave, this was explained by asserting that light-waves travels through a "light-medium," which was called the electromagnetic aether. This thinking led to a great deal of theory being developed to describe the relationships between light, electricity and magnetism in this "aether" medium.

Unfortunately, it turned out that the electromagnetic aether does not exist. Our best theories now require that there is no such thing as aether. Instead, the wave-like properties of light are explained by quantum mechanics.

The Pessimistic Meta-Induction

What do we conclude in the face of all these ill-fated scientific theories? For some, these examples motivate antirealism about the unobservables in science.

Antirealism is the view that we must be agnostic about truth when it comes to the unobservable aspects of scientific theories. All we can say is that the unobservables in our theories are useful instruments that happen to work reliably and successfully. But we do not have any good reason to say whether they are true or false.

Why be an antirealist? The history of science makes a pretty compelling case.

Intuitively, the idea is that after witnessing successful scientific theories fail time after time, we ought to suspect that our current theories will fail as well. Since this conclusion is both pessimistic, and an induction on the history of science, it is often referred to as the pessimistic meta-induction.

However, this is not by itself a good argument. You could ask everyone you meet for your entire life whether or not they're a major lottery winner, and only find people who are not. But this does not imply that major lottery winners don't exist.

However, the argument can be formulated more precisely as follows; this is what we will consider to be the pessimistic meta-induction.

Pessimistic Meta-Induction

This is a valid argument; in particular, it is a reductio ad absurdum, sometimes called an "argument by contradiction." And this is a more serious difficulty for the realist to contend with.

Genuine Success

One way out of the pessimistic meta-induction is to challenge premise 2. In particular, one might argue that past theories were not really as successful as they are being made out to be. here

For example, the crystalline sphere theory didn't predict much. Although it did provide a model of cosmology, it didn't experience anywhere near the same degree of predictive or explanatory success as modern theories.

Similarly, the humor theory wasn't particularly mature in its success. Although it perhaps had a few more explanatory and predictive successes, it was also extremely primitive and unsophisticated in what it could predict, especially compared to modern germ theory.

So, one realist response is to be more stringent in characterizing genuine success. The types of success that modern science has experienced is much harder to achieve, such as Einstein's prediction of light bending, and the Fresnel-Poisson prediction of a spot in the center of a shadow.

Thus, a first realist response is to characterize success as requiring both novel predictions and explanations as well as a mature theoretical framework in which to make such predictions and explanations.

Preservative Realism

Even when we are more careful about what we mean by success, some past theories are still just plain false. For example, Copernicus got it right that the earth goes around the sun, but incorrectly thought that the sun was the center of the Universe and that the stars are all fixed.

But here's an easy way to fix that. Suppose that we just focus on the successful parts of a scientific theory, and assert that they are approximately true. For example, what made Copernicus successful is the part of his theory that said the earth goes around the sun, not the rest of it.

The view that the successful parts of a scientific theory are what we should consider to be approximately true is called preservative realism, although Psillos refers to it as the "divide et impera" response. It is a simple, sophisticated form of realism in which we view science as constantly cutting out the unsuccessful parts of theories, while "preserving" the parts that are successful.

It's kind of like having a puzzle, in which we're keeping the pieces that provide a successful description, discarding those that don't, and sometimes adding more pieces when necessary.

The Case of Caloric

Is preservative realism correct? Or, are there examples from history in which even the successful parts were false in some scientific theory?

One potential challenge to the preservative realist story is the caloric theory of heat.

According to this theory, heat was a chemical element, just like oxygen or hydrogen. But the special role of caloric was to produce temperature. So, more caloric meant more temperature. Caloric was thus identified with heat.

There were two main principles about the nature of caloric formed the underpinning of the theory.

First, caloric was self-repelling. So, bunches of caloric didn't want to be near to each other.

This explained why a glass of cold water eventually warms up to room temperature in the summertime. The air in the room had a high density of caloric because it was warm. The ice water had a low density of caloric. So, the caloric in the air repelled away from itself into the glass, causing its temperature to increase.

Second, caloric had two states, sensible and latent. When in its sensible state, caloric increased temperature. But when in converted into a latent state, it did not affect temperature, but rather increased volume.

This explained why you get an increase in temperature when you compress a gas.

It was known, for example, that when you put water in a pipe under a lot of pressure, that it heats up. When this happened, hot bubbles would sometimes come to the surface. The explanation was that caloric was being squeezed out of a its latent state, in which it only effected the volume of the water, into a sensible state, in which it increased the temperature.

This idea led Laplace to make the novel prediction that a sound wave should heat up the air. Since the wave is continuously going up and down, the air should get hot as caloric gets squeezed from latent into sensible form. This led him to a dramatically correct prediction of the speed of sound in air.

So, we seem to have a genuine, mature, successful theory in the caloric theory. The only problem is that it is false.

Today, we know that caloric does not have two states. It is not self-repelling. In fact, caloric does not exist. Rather, heat is explained by the motion of particles.

What can the realist say to that? Can realism be saved? More details from the history of science may be required, but I leave it to you to decide.