Evolution is often described as a fact, and a theory. Evolution is a heavily overloaded term, with one definition being the fact that, "changes in the frequency of alleles in populations of organisms from generation to generation." This is a measurable fact, but not the usage of the word evolution that I am in interested in.

Wikipedia has several quotes on the matter, including the Webaters definition used in this questions title, but also includes clearly observational facts like the Earth revolves around the sun.

A fact is a hypothesis that is so firmly supported by evidence that we assume it is true, and act as if it were true. —Douglas Futuyma

There is no sharp line between speculation, hypothesis, theory, principle, and fact, but only a difference along a sliding scale, in the degree of probability of the idea. When we say a thing is a fact, then, we only mean that its probability is an extremely high one: so high that we are not bothered by doubt about it and are ready to act accordingly. Now in this use of the term fact, the only proper one, evolution is a fact. —H. J. Muller

Scientists most often use the word "fact" to describe an observation. But scientists can also use fact to mean something that has been tested or observed so many times that there is no longer a compelling reason to keep testing or looking for examples. The occurrence of evolution in this sense is fact. Scientists no longer question whether descent with modification occurred because the evidence is so strong. —National Academy of Science

Do scientists refer to any theories for which there is overwhelming evidence as a "scientific fact", other than in biological sciences?

Note: Good answers will include quotes from scientific organizations or prominent scientists using the term fact to describe the theory, and will include references to the theory itself.

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    $\begingroup$ the definition of observation and fact (which is not often used in scientific discussions in my experience) are messier than you might think. scientific proof is a messy business and different fields have different levels of evidence for publication, but they also have varying levels of application. There is in practice no standard definition of these words in 'science' or 'biology' that I have seen, certainly none is taught in graduate school. $\endgroup$
    – shigeta
    Jun 16, 2013 at 15:42
  • $\begingroup$ The use of the term "Law" (e.g., Newton's Laws of Motion, the Laws of Thermodynamics) seems comparable to the use of the term "fact". $\endgroup$
    – user1858
    Jun 16, 2013 at 20:25
  • $\begingroup$ @PaulA.Clayton, indeed. "Law" is an appropriate term, and is used often in Physics. I am interested in the supposed misuse of the term "fact" that scientists supposedly use according to the NAoS and others in fields other than biology. (Bonus points for not using climate scientists "fact" of the majority of warming is human caused in reference to the AGW theory.) $\endgroup$
    – user1873
    Jun 16, 2013 at 20:44
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    $\begingroup$ user1873-> Isn't the point of @PaulA.Clayton that the use of 'Law' in physics is an example from a field other than biology of a similar expression to denote a theory (in that both 'law' and 'fact' indicate a high degree of 'truth'). If you accept that 'scientific law' == 'scientific fact' then you have at least one example. $\endgroup$ Jun 18, 2013 at 14:32
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    $\begingroup$ AFAIK, there are (1) evolution, (2) the Theory of Evolution and (3) Darwin's Theory of Evolution. (Other 'theories of evolution' could be counted.) The three are not equivalent. It seems that that you are asking about one of the three, but it's not clear which one. $\endgroup$ Oct 12, 2014 at 9:56

1 Answer 1


Your question really applies to any scientific theory, not just evolution.

The problem is that, outside of the scientific world, a theory is often thought as something unproven, an unproven guess if you wish. This is not true for scientific theories, which, instead are confirmed by experimental data.

The OED defines theory as:

A scheme or system of ideas or statements held as an explanation or account of a group of facts or phenomena; a hypothesis that has been confirmed or established by observation or experiment, and is propounded or accepted as accounting for the known facts; a statement of what are held to be the general laws, principles, or causes of something known or observed.

and fact as:

Something that has really occurred or is actually the case; something certainly known to be of this character; hence, a particular truth known by actual observation or authentic testimony, as opposed to what is merely inferred, or to a conjecture or fiction; a datum of experience, as distinguished from the conclusions that may be based upon it.

So, essentially we start with an unproven idea, we corroborate it with facts (=experimental data) and we define our theory accordingly to facts.

So, why don't we say that evolution is true or that it is a dogma or that it is a fact?

Well, that is not how science work. Evolution is a scientific theory, and it always will be. Maybe (hopefully, dare I say) in 50 years from now new evidence will have make us change the theory of evolution, at least some bits of it.
Luckily evolutionary biology is not stuck at the time of Darwin and genetics is not stuck at the time of Mendel. Many new pieces of evidence have made us improve the theories that they initially defined.

Does that mean they were bad scientist or that they experiments were faked? Not at all! Simply as their knowledge was more restricted than ours, they could not formulate a more advanced theory. Mendel's experiments still hold true, but now we know that transmissions of characters is not as simple. He was just looking at a special case. And his experiments still fit in our current theory, and will have to fit in any future one.

A scientific theory is defined by its falsifiability, that is

the trait of a statement, hypothesis, or theory whereby it could be shown to be false if some conceivable observation were true. In this sense, falsify is synonymous with nullify, meaning not "to commit fraud" but "show to be false".

The famous black swan example by Karl Popper (see e.g. Conjectures and refutations, 1963, chapter 11.6), can be exemplified by (source: Wikipedia):

One notices a white swan. From this one can conclude:
At least one swan is white.
From this, one may wish to conjecture:
All swans are white.
It is impractical to observe all the swans in the world to verify that they are all white.

Even so, the statement all swans are white is testable by being falsifiable. For, if in testing many swans, the researcher finds a single black swan, then the statement all swans are white would be falsified by the counterexample of the single black swan.

So, our theory of the white swan is accepted as "temporary true", as long as a black swan is not found. Obviously the more white swans we count (i.e. the more experimental evidence we have), the stronger our theory will be. But it will never be true.

I would like to finish quoting Richard Feynman:

It is scientific only to say what is more likely and what less likely, and not to be proving all the time the possible and impossible.


We never are right, we can only be sure we're wrong

I suggest to watch the full video of his fantastic lecture on scientific method on YouTube.

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    $\begingroup$ An biologist, a physicist and a mathematician are on a train in Scotland. The biologist looks out of the window, sees a black sheep standing in a field, and remarks, "all sheep evolved in Scotland to be black because selective forces killed off the white sheep." "No, no, no!" says the physicist. "Only some Scottish sheep are black." The mathematician rolls his eyes at his companions' muddled thinking and says, "In Scotland, there is at least one sheep, at least one side of which appears to be black from here some of the time." $\endgroup$
    – user1873
    Jun 16, 2013 at 17:03
  • $\begingroup$ @user1873: you seem to have misread my answer. I never said scientists use fact in reference to theories. I am saying that a scientific theory, or a scientific model, is based on facts, that is experimental proofs. One, however, can never be certain a theory it is true (instead one can be certain a fact is true). So, if a scientist or a journalist or anyone else uses "theory" and "fact" interchangeably... well, they are doing it in an incorrect way as the two terms do not mean the same thing. $\endgroup$
    – nico
    Jun 16, 2013 at 17:47
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    $\begingroup$ @nico In your answer and comments you write that a theory is an interpretation of facts, and that one can be certain a fact is true. This is however (as you probably know) a very simplified view. More modern descriptions of the scientific method all highlight how observations cannot be simply decoupled from theory (see e.g. Lakatos in Criticism and the Growth of Knowledge and Kuhn's The Structure of Scientific Revolutions, or, in more extreme form, Feyerabend's Against method). Therefore, you can just as well argue that observations and "facts" are theory dependent. $\endgroup$ Jun 18, 2013 at 12:08
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    $\begingroup$ @nico Well, neither observations nor theories will change the physical reality, so no difference there. While I agree in practice that observations and theory have a different standing, with observations being closer to "actual facts", I still think that the point of theory-dependent observations cannot be so easily written off. A classical example would be astronomy, where raw 'observations' can consist of photon recordings from a distant star. For this raw 'observation' to be made into an observation (fact?) that has bearing on a particular hypothesis (e.g. dealing with ... $\endgroup$ Jun 18, 2013 at 14:59
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    $\begingroup$ contd: ... the composition of the star) an entire body of work must be assumed. Auxilary theories and methods that has to be assumed true could include recording equipment, diffraction of light in the atmosphere and in space, how electromagnetic radiation is emitted etc. The point being that these observations don't exist except in relation to a complex network of theories. The astronomy example might be seen as extreme, but I can easily envision many similar situations in biology. But I probably digress from the main topic.... $\endgroup$ Jun 18, 2013 at 15:01

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