Recently, I met a college student whose professor had told her that it is a fact that all living organisms are descended from a single life form. I quickly informed her that her professor was wrong-- that is not a fact, it is one hypothesis that stems from the theory of evolution. However, lest this student use the idea that evolution is “only a theory” to dismiss it entirely without giving it an honest look, I hastened to add that a theory, or even a hypothesis, is far from being a guess. I later wished that I had had time to explain the difference between fact, hypothesis, and theory-- for all of these have definitions in science, which differ from how they are often used in the vernacular. So, in regret that I cannot explain it to her in person, I will explain it here, hoping it will be helpful to someone else whose professor has made or will make the same mistake.
I love what one of my old ecology textbooks says about this: It defines facts as “particular truths of the natural world” and then goes on to state, “The notion of truth is a profound one that philosophers discuss in detail and scientists just assume is simple. Truth consists of correspondence with the facts.” (Krebs 2001:13, 14)
This sounds circular, but it’s really just saying that facts are things that are true. Furthermore, facts are true whether or not we know or understand them correctly:
“[scientists] make observations, which may be faulty, and consequently every observation is not automatically a fact.” (Krebs 2001:13)
The UC Berkeley “Understanding Science” website defines it this way:
“Fact: Statement that is known to be true through direct observation. Since scientific ideas are inherently tentative, the term fact is more meaningful in everyday language than in the language of science.”
In other words, in the actual doing and reporting of science, “fact” isn’t a term we use a lot. (Also note an idea we discussed in a previous post, that scientific knowledge is tentative.) Here is a very relevant example:
"[In] scientific thinking ...we can only be completely confident about relatively simple statements. For example, it may be a fact that there are three trees in your backyard. However, our knowledge of how all trees are related to one another is not a fact; it is a complex body of knowledge based on many different lines of evidence and reasoning that may change as new evidence is discovered and as old evidence is interpreted in new ways. Though our knowledge of tree relationships is not a fact, it is broadly applicable, useful in many situations, and synthesizes many individual facts into a broader framework. Science values facts but recognizes that many forms of knowledge are more powerful than simple facts.” (UC Berkeley link above)
Hm, what an intriguing statement! Knowledge that is more powerful than facts? Read on!
Once a scientist has some observations in hand, he/she formulates a possible explanation consistent with everything else he/she already knows. This possible explanation is a hypothesis, and it must be stated in a way that allows it to be tested by experiment or further observation. A hypothesis makes predictions about future observations, and if these predictions are actually observed, the hypothesis is supported. Observations contrary to the predictions would help refute the hypothesis. Remember that testability and falsifiability are flip-sides of the same coin; for a hypothesis to be scientific, it must be possible in principle to gather evidence that would support or refute it. (See more on testability, and see “Falsifiable” here for further clarification of these terms.) Even so, we can never prove or disprove a hypothesis with absolute certainty.
Again, the Understanding Science project summarizes well (they make my job a lot easier!):
“Theory: In science, a broad, natural explanation for a wide range of phenomena. Theories are concise, coherent, systematic, predictive, and broadly applicable, often integrating and generalizing many hypotheses. Theories accepted by the scientific community are generally strongly supported by many different lines of evidence-but even theories may be modified or overturned if warranted by new evidence and perspectives.”
And (still quoting from the above link) here is a further point I wish to emphasize:
“Occasionally, scientific ideas (such as biological evolution) are written off with the putdown "it's just a theory." This slur is misleading and conflates two separate meanings of the word theory: in common usage, the word theory means just a hunch, but in science, a theory is a powerful explanation for a broad set of observations. To be accepted by the scientific community, a theory (in the scientific sense of the word) must be strongly supported by many different lines of evidence. So biological evolution is a theory (it is a well-supported, widely accepted, and powerful explanation for the diversity of life on Earth), but it is not "just" a theory.”
Nor, I would add, is it as simple as fact. A theory, by definition, is an overarching, explanatory idea that rests upon numerous facts and hypotheses. It is a BIG, HUGE, elegant idea that squares (at least so far) with many facts we have in our possession… insofar as we can tell that they are facts. A theory is also generative, spurring new hypotheses and avenues of research in an organic, ongoing process that is never complete. And, as with any scientific idea, some aspects of a theory may turn out to be right, and some may turn out to be wrong-- and we can’t tell the difference yet. Moreover, a theory can remain well-supported and valid as a research scaffolding, despite the incompleteness of some of its sub-elements. For example, it may turn out that we have correctly understood the basic laws of inheritance that lead to genetic changes in a population over time, while a complete family tree linking all life forms to a common ancestor remains elusive. Both of these ideas fit under the umbrella of “The Theory of Evolution,” and either of them may (and probably will) be modified as our understanding… er, evolves. Hence, no one who understands the breadth and dynamic function of theory would be so simplistic as to equate it with fact.
So… the next time someone tells you that evolution is a fact, you can tell them with great enthusiasm that on the contrary, evolution is not a fact; it is much more than only facts-- it is a theory! How exciting is that?! (You may want to explain this using large hand gestures, indicating that, in science, theory is a BIG thing compared to fact.)
And then, if they ask you whether you believe in it, you can tell them that while a theory is well supported by multiple lines of evidence, putting absolute faith in a scientific theory is not scientific. No one can fault you for refusing to commit your final opinion, because science does not have the final word. And that’s a fact.