by David Geelan  |  28 March 2019  |

“Evolution is a fact” is something we tend to hear from one side in debates about origins. I’d argue that this statement reflects a misunderstanding of the roles of facts, theories and laws in science.

At the same time, from the other side of those same debates, we tend to hear “evolution is (just) a theory,” which is equally unfortunate as a way of thinking about what “theory” means in science.

I want to, as clearly as I can, briefly outline the meanings in science of the terms “fact,” “theory” and “law,” and to explain why a theory, no matter how well supported by evidence, never turns into a fact.

Evolution is, in Seventh-day Adventist circles, a controversial theory, and therefore a bit awkward to use as an example for this discussion, since it brings in strong emotions and strongly held views on the part of readers. (And, just quietly, I’m a physics guy, not a biology guy.) So, instead, I’ll use the example of gravity.

Here is a fact about gravity: close to the surface of the earth, if any object (that has mass) is unsupported, it will accelerate toward the center of the earth with an acceleration of about 32 feet per second per second, or about 9.8 meters per second per second. If you have an object and a stopwatch handy right now (and your phone probably has a stopwatch function), and have done a little high school physics, you can test this fact.

In science, “fact” is used to refer to a single piece of data, the result of a measurement. Other facts about gravity include the fact that it decreases in strength as we move away from the center of the earth, and that every object that has mass exerts a gravitational force on every other object that has mass. With sophisticated-enough instrumentation, all these facts can be measured and expressed in numbers.

Not all of science is physics, though, as my students often remind me. It’s a fact in chemistry that sodium chloride (table salt) has a cubic lattice structure between its atoms, and a fact in biology that living things contain DNA, and a fact in geology that most rocks contain a lot of silicon dioxide.

Let’s leave “theory” on the side of our plate for the moment, because it’s the most complicated, and talk about “law.” In science, a law is a mathematical relationship between quantities. The most famous law in science is probably Einstein’s E = mc2, which describes the relationship between matter and energy.

The key law in gravity, which was formulated by Isaac Newton (and I do apologize for those who find equations challenging!), is F = (Gm1m2)/r2. In words, it says that if there are two masses, m1 and m2, a distance r apart, the force F between them is given by this law, where G is called the “universal gravitational constant.”

A law is powerful because it makes a relationship clearer. A couple of paragraphs ago I said that the force decreases with distance, but the law gives more detail. It shows that the force decreases with the square of distance: if the objects are twice as far apart, the force is only one-fourth as great.

A theory is a human mental creation that explains facts and has withstood the test of experiment. This view of the nature of science and of theory is owed to philosopher of science Karl Popper. A theory in science has descriptive, predictive and explanatory power. That is, a theory describes the world as we see it and experience it. It allows us to reliably predict how the world will behave in the future in a particular set of circumstances, and it explains why the world is as it is. If we make a prediction using a theory, and then conduct the experiment and the prediction fails – the world does not behave as the theory leads us to expect – then Popper would say the theory has been “falsified” and should be discarded. The theories that make up science at any given moment are the ones that have been tested many times and have never been falsified. Einstein neatly summed up Popper’s perspective: “No amount of experimentation can ever prove me right; a single experiment can prove me wrong.”

A theory is not held to be “true” in any final sense under this view: at best, it is the most powerful theory available that explains the greatest number of facts and has not been falsified. A new experiment may yet be conducted that will falsify it, and if that occurs the theory will need to be discarded and replaced with a better one.

The first – and longest-lived – theory used to explain our experience of gravity was proposed by Aristotle. He suggested that things in the universe have their “natural station,” the place where they belong. Things mostly belong on the ground – even birds – so when we lift them above the ground they are being lifted out of their natural state, and if they are not prevented from doing so, they will return to it. If we lift a book from the floor to a table, it is out of its natural place, and if the table were not there to prevent its doing so, the book would return to its natural place on the floor.

Aristotle’s theory of gravitation applied only on Earth, since he also believed that the heavens were a different domain from Earth, with different rules and processes. The contribution of the next great theorist of gravity, Isaac Newton, was to take the rule that was used to explain how things move on Earth and apply it to objects in space such as the moon and the planets.

Johannes Kepler had created rules that described the motion of the planets in purely mathematical terms, but did not explain why the planets moved the way they did. Most times laws are derived from theories, but we could argue that Kepler’s laws were not drawn from a theory. They had descriptive and predictive power – Kepler could tell you when the next eclipse would come – but not explanatory power.

Newton developed the theory that any two objects with mass exert a force on each other, and – crucially – that this is true in the heavens as well as on Earth. The same force that caused his (probably apocryphal) apple to fall from the tree to the ground explained the motions of the heavenly bodies. Newton had a theory, not just a law, because in addition to description and prediction, it was capable of explanation.

Newton’s theory of gravity still works well for everything we encounter in everyday life, but for much more extreme environments, such as near the event horizon of a black hole, it breaks down. For those specialized contexts, it has been replaced by the theory of general relativity proposed by Albert Einstein. The mathematics gets very complex very quickly, but in words, Einstein’s theory can be stated as “matter tells space how to curve; space tells matter how to move.” Gravity is explained, not as a force between objects, but as mass causing curvature in the local space, which then causes mass to move differently.

Einstein’s theory is considered “better” than Newton’s because it is more universal – it can be applied everywhere in the universe, whereas Newton’s theory breaks down in some situations.

All three of the theories described – Aristotle’s, Newton’s and Einstein’s – explain the fact that a dropped book or apple will fall toward the ground. Aristotle’s does not have an associated law – a mathematical statement about how rapidly the apple will fall – while Newton’s theory does include a law. Einstein’s theory also includes laws, but the mathematics are too complex to go into here.

I hope these examples have helped to explain why a theory can never turn into a fact or a law, no matter how much evidence it has behind it. These are three different things with different qualities, each important in science.

I suspect that someone who says “evolution is a fact” is using the word “fact,” not as a scientist would, but in the everyday sense of “not fiction.” They mean that a textbook on evolutionary theory in the library would not be placed with the novels and short stories, but with the other books that contain true information about the world. It’s probably still an unfortunate usage, though, since the claim being made falls within science, so the language used ought to be the careful language of science.

Evolution is a theory. It is one that explains, not one fact, but an enormous variety of facts about the diversity and the characteristics of life on Earth. It is a theory that was proposed more than 150 years ago, and it has been tested in a wide variety of ways. The fundamental concepts have not been falsified, but significant elements have been changed and updated. Darwin did not know about genes when he wrote On the Origin of Species, for example, nor did he know about DNA. He did not have access to the vast array of data about living things that modern scientists can draw on. The modern evolutionary synthesis includes the recognition that gene transfer plays a much greater role than previously thought, for example, so that less of the “heavy lifting” of generating new characteristics must be borne by mutations.

When people say “evolution is (just) a theory,” they are drawing on the common, everyday use of the word, rather than the scientific use. We say “I have a theory” when we mean a guess, a hunch, an untested brainwave. Saying “evolution is a theory” is thought of as a way of saying that it is unsupported, held without evidence, untested. These same people would tend not to say “gravity is (just) a theory,” although in scientific terms, gravity is indeed a theory. Or, at least, there are multiple theories of gravity that explain the facts of gravity, some of which provide mathematical laws, and Einstein’s is currently the best theory we have.

Theories do change, and Einstein’s theory may well be replaced by an even more powerful one in the future. There are interesting problems at the boundaries between general relativity and quantum theory, for example, that may revolutionize our understanding in the future. But a book will still fall from a table, and an apple from a tree. A change in the theory of gravity will not enable us to suddenly, unaided by technology, safely walk off the roof of a tall building and just float. The facts of gravity will remain the same even if the theory used to explain them changes.

The same is true of evolutionary theory. It has changed in the past and is likely to continue to change. Alternative candidate theories, including special creation and intelligent design, already exist, and already claim to explain the same facts. When the theory changes, the facts will not change. The DNA that forms the genetic blueprint for a jellyfish will not suddenly begin to produce a lion instead.

It’s probably a discussion for another article but to date, the alternative candidate theories – special creation and intelligent design – have not demonstrated descriptive, predictive and explanatory power in the same ways and to the same extent as the modern evolutionary synthesis. There are efforts to make these demonstrations, which will continue to be tested against the facts of biology.

I hope this brief discussion has been helped you to understand the meanings of the terms “fact,” “theory” and “law” in science, and to be aware when these terms are being used confusingly in discussions around origins as well as other scientific topics such as vaccination, genetic modification and climate change. If we can communicate clearly and accurately, and go forward together in good faith, we have more chance of finding our common ground.

David Geelan is Sue’s husband and Cassie and Alexandra’s dad. He started out at Avondale College and has ended up (so far) as an Associate Professor of Science Education at Griffith University on the Gold Coast, Australia.

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