I recently answered to this question on Biology.SE.

The question asks:

  1. Is evolution a fact?
  2. Is it true that human and chimpanzee evolved from ape?

In my answer I started with a tiny bit of terminology, defining the terms theory and fact as used in science and in the popular culture. I did not want to go into the details because

  1. although I understand these concepts quite well I think, I am not a philosopher of science (but a biologist) and am therefore not as qualified as you guys might be to explain this terminology

  2. I don't think advanced definitions of these concepts should be provided on Biology.SE but rather on Philosophy.SE

Most comments concerned the terminology. I agreed with them and edited my question accordingly but many people keep commenting (either direclty under the question or in the chat room) for questions of terminology.


I am looking for an easy-to-read but rather advanced explanation for a non-philosopher and non-scientist document defining the terms:

  • fact
  • theory
  • hypothesis
  • evidence
  • prediction


I googled my question and got a few hints but had hard time to find a good authorative answer that I could link to my answer on Biology.SE.

How to answer

One can answer to the question direclty here or just suggest a link that would correspond to my needs.

3 Answers 3


Since you're asking as a biologist, let me give a biologist's answer. Stephen Jay Gould answers this in one of his books. Gould was a professor of evolutionary biology at Harvard, died about 13 or 14 years ago, but his books are still available and popular. He came up with the theory of evolution called punctuated equilibrium.

He dealt with this subject in his book Hen's Teeth and Horse's Toes: Further Reflections on Natural History, specifically Chapter 19: Evolution as Fact and Theory. He speaks on what is a fact and what is a theory and what is a hypothesis.

In this chapter he writes:

The basic attack of modern creationists falls apart on two general counts before we even reach the supposed factual details of their assault against evolution. First, they play upon a vernacular misunderstanding of the word 'theory' to convey the false impression that we evolutionists are covering up the rotten core of our edifice. Second, they misuse a popular philosophy of science to argue that they are behaving scientifically in attacking evolution. Yet the same philosophy demonstrates that their own belief is not science, and that 'scientific creationism' is a meaningless and self-contradictory phrase, an example of what Orwell called 'newspeak.'"

In the American vernacular, 'theory' often means 'imperfect fact'--part of a hierarchy of confidence running downhill from fact to theory to hypothesis to guess. Thus, creationists can (and do) argue: evolution is 'only' a theory, and intense debate now rages about many aspects of the theory. If evolution is less than a fact, and scientists can't make up even their minds about the theory, then what confidence can we have in it?...

So, first you have to differentiate between evolutionary fact and evolutionary theory. The fact is that when we dig in the earth we find the remains of animals that are no longer on this earth and we have no written record of their existence. Second, as we dig deeper and deeper, the forms of those animals become simpler and simpler. Third, through established scientific methods (i.e., radiocarbon dating) we can establish the times that these animals existed. These are evolutionary facts. These are the world's data.

Gould further states:

Evolutionists make no claim for perpetual truth, though creationists often do (and then attack us for a style of argument that they themselves favor). In science, 'fact' can only mean 'confirmed to such a degree that it would be perverse to withhold provisional assent.' I suppose apples might start to rise tomorrow, but the possibility does not merit equal time in a physics classroom...Evolutionists have been clear about this distinction between fact and theory from the very beginning, if only because we have always acknowledged how far we are from completely understanding the mechanisms (theory) by which evolution (fact) occurred. Darwin continually emphasized the difference between his two great and separate accomplishments: establishing the fact of evolution, and proposing a theory--natural selection--to explain the mechanisms of evolution.

Evolutionary theory is how we explain these physical facts that we have dug up. An evolutionary theory must be able to explain the evolutionary facts that we have been presented with (dug up). If you present an 'evolutionary theory' that does not support the facts, then it is not a theory it is an unfounded conjecture - a guess. Calling a conjecture a 'theory' does not make it a 'theory' - as can be seen from Prof. Gould's quote above.

Scientific evolutionary theories have some variations (witness Gould's own punctuated equilibrium theory vs classical slow or gradual evolution), but they are all based on the facts that have been literally dug up.

Creationist 'theory' or better guess or conjecture, does not base itself on the evolutionary facts that have been dug up.

As far as 'evidence', this is a term that is not a part of the scientific method, and has a generic meaning that observations have been made but those observations have not been firmly established as facts.

A theory rests on being able to 'predict' future events.

  • Fact: I consider it the term on your list which is most difficult to define. Because it is a basic term. As a working definition I propose:

A fact is a component of reality.

Facts are real situations as opposed to possible situations and impossible situations. Remember also the beginning of Wittgenstein, Ludwig: Tractatus Logico-Philosophicus:

                  1.2 The world divides into facts.
  • Theory:

A theory is a set of terms, statements, predictions or retrodictions, proofs or confirmations, and open issues.

A theory does not belong to the domain of reality like facts, but to the domain of ideas.

  • Hypothesis:

A hypothesis is a conjecture which has not yet been falsified.

In formal sciences like mathematics a hypothesis is an unproved claim which seems plausible to the experts.

  • Evidence:

Strong confirmation of a conclusion derived within a theory.

  • Prediction:

A statement derived as part of a theory about a future event.

Predictions should be testable, so that they will be either confirmed or falsified.

  • Thank you! Would it be more correct to say A theory is a set of terms, statements, predictions or retrodictions, proofs or confirmations, and open issues that are strongly supported by many evidences?
    – Remi.b
    Nov 17, 2015 at 0:21
  • I preferred to not include evidence into the definition. What about string theory?
    – Jo Wehler
    Nov 17, 2015 at 0:29
  • I don't know much about string theory! In the way you defined theory, it doesn't sound like it implies that we have any level of confidence that the body of terms, statements [etc..] are true. It seems that as long as the "story is long enough", then we have a theory regardless of empirical testing.
    – Remi.b
    Nov 17, 2015 at 0:38
  • Investigating the question a little further I discover that definitions of theory seem to vary a lot (see here) though.
    – Remi.b
    Nov 17, 2015 at 0:42
  • Given a list of definitions like this, I'm curious: do you feel these definitions are particular to one flavor of science (such as the one advanced by Popper), or have these definitions held relatively consistent as science evolved over the years?
    – Cort Ammon
    Nov 17, 2015 at 4:42

Theory is just a formal statement which includes the known limitations and the assumptions.

Fact: As said by Jo Wehler, any documented event as it is without any inferences could be called a fact (which cannot be falsifiable). However, I think mathematical theorems can also be considered tantamount to facts because a theorem is proven to be true for all conditions. There are conjectures which say that a certain statement appears true for a lot of tested conditions but its trueness is not still established for all possible conditions. cannot be considered facts.

Hypothesis is a testable statement made prior to testing. It is not even a conjecture.

Evidence is a set of data that is necessary and sufficient to establish a theory.

Prediction is an set of unknowns calculated for a set of known parameters, using a given model. As mentioned by Jo Wehler, predictions should be testable.

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