Fact or Theory?

by John P. Pratt
©1998 by John P. Pratt. All rights Reserved.

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It is extremely important to distinguish between facts and theories in science, and in every other subject also, because facts usually remain the same and theories often change. They are not always easy to differentiate, and even scientists forget to do it. And the people who write science textbooks nearly always forget to do it. So I'll try to give you some guidelines so you can do it yourself, because twenty years from now the facts will be the same, but the theories may have changed a lot. I know because I've had to relearn some theories since I taught this class 25 years ago, but the facts are the same. For example, the universe has doubled in age since I was a boy, which gives you an idea of just how old I am. In this class, we will try to spend about 2/3 of the time on facts, so that you won't need to relearn the entire class when the theories change. Of course, we can't ignore the theories altogether, because they are our best explanations of the facts; it's just that we shouldn't consider them to be absolute truth. When some people see a scientific theory disproven and replaced by another, they lose confidence in all of science, which is a big mistake. That is how science progresses, and it is a wonderful way to learn about our world. So let's begin by reviewing the scientific method, to see just how facts and theories interact.

The Scientific Method

There are essentially three steps to the scientific Method, although some authors break it in to more. These steps are to observe something, then to try to explain it, and then use your explanation to predict future observations. Let's look at these steps more closely.

1. Make Observations.

Let's suppose we were interested in this marvelous world in which we live enough to actually look at it once in a while. Suppose that in the early morning light, you are walking with two friends and notice that there is a halo around your shadow, glowing in the dew on the grass. You notice also that there is no such halo around the shadow of your friends. Being curious about the world, you ask yourself, "What causes the halo?" As we become more aware of the universe around us, we will find ourselves asking such questions often.

2. Propose a Theory.

Try to explain what you've observed. The man who first wrote up this observation, proposed the theory that it was because of his superior intelligence. Before reading on, do you think he is right? If not, pause for a moment and try thinking of your own theory. How could you test either his theory or your own theory to see if it might be right?

3. Use the Theory to Predict Future Observations.

The heart of science lies in this third step. Having your theory, use it to predict the outcome of a future observation. This is the "testing" part of science. In the current example, you could ask your friends if they see anything unusual about any of the shadows. (If you don't tip them off what your theory is, you'll get a more unbiased answer.) Suppose they both answer that each sees a glow around her own shadow. What does this tell you about your theory? What does it tell you about the man's theory with his supposed superior intelligence?

Falsification. An important point here is that if the prediction fails then the theory must be discarded or changed. If it was really his superior intelligence, then the prediction would be that the two friends would either agree that only his shadow had the aura, or perhaps they would see no aura at all (because of their inferior intelligence!). The simple observation that each sees his own aura falsifies his theory because not all three can have an intelligence superior to both the others.

These three steps are usually repeated over and over, often refining the theory after each set of new observations or experiments, with increasingly difficult testing hurdles for the theory to overcome. The most valuable theories are those which make precise and risky predictions, which could easily disprove the theory if they failed.

Repeat the Three Steps Until Satisfied. If your theory passes the first falsification test, then think of another experiment to test another aspect of the theory. The idea of science is to repeat the three steps over and over until you are convinced you have a theory good enough to correctly predict the outcome of experiments in a wide variety of situations. To do this, scientists like to use "controlled" experiments when only one thing changes each time. In this example, you might try looking for a halo at a different time of day, or without the dew, or on something besides grass. Each time your theory should make a measurable prediction.

Unscientific Theories. If your theory makes no prediction, then it cannot be tested and hence it is not scientific. It still might be the correct explanation, it is just not scientific because the scientific method cannot be used to falsify it. There are many theories out there which cannot be tested, masquerading as scientific theories in order to have credibility. Be on the lookout for them.

Truth. Note that you cannot prove any theory to be true. You might think up a thousand totally different tests to try to disprove the theory, and it might pass every one. Does that mean it is "true"? No, because the 1,001st test could prove it false. While scientific theories are never supposed to be considered to be absolute truth, some have passed so many tests that they are called "laws." For example, we will learn Kepler's laws, and Newton's laws. A scientific law is like a theory that has been inducted into the "Science Hall of Fame." But even then it might have to be modified. Einstein found some corrections even for Newton's laws, but they are normally far too tiny to even be able to measure.

Everyday Scientific Method. You probably use the scientific method everyday more than you might have suspected. If your car stops, you first think it might be out of gas, but the fact is, the gauge says you have plenty. After you try several things which fail (falsifying those theories), you replace the gas filter and suddenly it works again. You haven't proven that it was the gas filter, but you'll believe it was unless something else convinces you otherwise (such as it quitting again in a similar manner.)

TV commercials are filled with suggestions for you. You observe that the guy who uses this toothpaste has a brighter smile and the girls all chase him. So you buy some on the theory that they'll soon be after you. But even if your date-acceptance-rate does measurably pick up, it may still be hard to trace it to the toothpaste; it might be simply that you are smiling more! That's why scientists try to have controlled experiments. But at least we can think about what we are doing anyway in a more scientific way.

A lot of times we just do the first two steps of the scientific method. We observe something, form a theory, and then simply assume the speculation is true. Maybe your girlfriend doesn't return your calls as quickly, so you decide she doesn't like you anymore. Or maybe she got caller ID. Before "jumping to conclusions," try thinking of ways to test your explanations of observations.

Usefulness of Science. It is the predictive power of science that is so powerful, and which has led to much of the marvelous technology we now enjoy. When we know the law of gravity, we can build bridges and even send rockets to the moon. Without the ability to predict future results, we do not have science, we have only speculation about what happened in the past. Perhaps the greatest achievement in chemistry was the periodic table, which predicted the existence and properties of several elements before they were discovered. Science is truly wonderful in what it has given us, and the rock on which it is founded is the scientific method.

The "Rest of the Story." I can't resist telling you how the true example above about the glowing shadows comes out. One of the first scientists to investigate this phenomenon (called "heiligenschein" = "holy light") proposed that the droplets on the grass act like little lenses which focus the light onto the leaves of grass, which reflects the light back through that same lens into a beam back toward the observer. That explains why it is only seen around the shadow of the observer, because it is directed back toward the sun. That explanation was immediately discredited by someone who merely asked the question" "If the light reflects off the leaf, then why isn't it green?" No one could answer that, and one entire century went by and no one could offer a better theory. Finally someone asked, "Well, is the light green?" He did some precise color measurements and sure enough, the light was green, and that first theory is now accepted. So the science in this area was halted for a hundred years because scientists didn't actually do an experiment to test it!


The word fact can be used several ways, but in general in science, "facts" refer to the observations. They are best when they are repeatable observations under controlled conditions, such as "It is a fact that the speed of light is constant in a vacuum." This is the part of science which will be the same a century from now, unless more precise measurements show otherwise.


The theories are the explanations proposed in step two of the scientific method. Usually the word "theory" is reserved for more than a first attempt, which might be called a "hypothesis." A theory usually has already survived several falsification attempts, and is pretty well accepted. However, I'll use the word theory to mean any explanation of observations.

Thus, by separating facts from theories, I mean distinguishing between observations and explanations. When you hear the news, "The Dow Jones took a plunge today because of fears about the Asian crisis," is that fact or theory? It was half and half. The fact is that the market went down: that is an observation which was measured. But who knows what drives the market? The matter-of-fact statement that it was caused by such fears could be anything from one reporter's speculation to a general consensus of market analysts. In any case, it remains only a theory. No one will separate the facts from theories for you; the trend is to present everything as truth.


Let's take just a moment to talk about truth. If science can never prove a theory "true," then truth really has no place at all in science. By "truth" I mean what is "really" going on. Truth has to do with ultimate causes, which are nearly always extremely elusive and beyond the realm of science. Science deals with theories, usually mathematical, which predict outcomes of experiments. For example, if we drop a rock off a cliff, the law of gravity combined with theories of air resistance and other forces can be used to calculate just how long it will take to hit the ground, and how fast it will go, etc. But science does not answer the question of just exactly what gravity is, or why things fall. It just states that given certain conditions, they will fall. In general, science answers questions like "how," "when," "where", but never "why" in the ultimate sense.

As an example of the interplay of the three concepts of observations, theories and truth, consider the courtroom. The observations may be that a man was seen shooting a gun and that the person hit by the bullet died. The theory may be that it was cold-blooded murder, but the truth may be that it was self-defense. Truth tends to be invisible and hidden, such as someone's motives, whereas observations are usually visible. Courts are very interested in truth, where the motive (the ultimate cause) for actions is given considerable weight. The distinction between first-degree and second degree murder is based on intent. Motives are not as yet observable in science, and hence are beyond science.

Try Replacing the Word "Fact"

The word "fact" has several meanings, which can be very confusing. In popular useage it can mean either "observation," "theory," or "truth." As an example of each, one can say, "it is a fact that every time I have dropped this ball, it fell to the ground." That is what has been observed so far, and the word "fact" can be replaced with "observation." One can also say, "it is a fact that every time I have dropped this ball, gravity pulled it to the ground." Even though this statement appears very similar to the first, "gravity" really refers to a theory proposed to explain why the ball is observed to fall. Finally, if one so thoroughly believes that the theory of gravity is really "true," he could replace "a fact" with "true," which would take the meaning beyond science into the realm of his personal convictions.

This confusion can often be avoided by always replacing the word "fact" with "observation," "theory" or "truth," whichever seems to convey the intended meaning best. Remember that if the meaning is "observation," then it is as fallible as the observer. If it is a "theory," then it also could be disproven someday. If it is claimed to be "truth," then it is a statement of the personal conviction of the speaker, which is outside the domain of science.

What to do

There are really two things I'm suggesting that you actually start to do.

Use the Scientific Method Daily. First, try making a practice of using the scientific method on a daily basis. You already formulate explanations of things you notice. The main step to add is some test with predicted results to test your theory. I do programming for a living. When I discover a "bug" in a program, I come up with a theory of what may be causing the problem. Before I test my theory by making the proposed correction, I always predict on what the result will be. I've noticed that other programmers will sometimes just try different things almost randomly to see what might be the result. I've found it helps me much more to always have a reason for the change, which is to test my current theory.

Separate Observations from Explanations. Secondly, when you hear the news, or see advertisements, try distinguishing between facts and theories. This can really be a fun exercise, and it is really instructive. It should be useful to you no matter what your major is. Maybe even get in the habit of asking for case studies, or articles which discuss just how something is known. Hopefully, I will never be upset if you ask, "Just how do you know that?" That is what science is all about. And I'll be expecting that of you. By the end of this course you should know the experimental evidence which caused even the ancients to conclude that the earth is spherical, and that most galaxies are rushing away from ours.