When science denialists say "just a theory", they are roundly chastised. Science educators (teachers, reporters, essayists, etc) are united that the word theory has a strong requirement:

  • Berkeley: "a theory (in the scientific sense of the word) must be strongly supported by many different lines of evidence."
  • Smithsonian: "A theory is a well-substantiated explanation of an aspect of the natural world that can incorporate laws, hypotheses and facts."
  • Wikipedia: "It is well-supported by many independent strands of evidence, rather than a single foundation."

However, physicists hold to a completely different view:

  • Dr Lubos Motl says a theory is a detailed & cohesive explanation, even if it only applies to "gedanken worlds" (thought experiments that don't match the real universe).
  • Physics.SE: in order for something to be called a "law," there must (or at least should) be experimental evidence supporting it. There is no such requirement to be called a "theory."
  • Physics.SE: An open-ended program you can publish new fundamental papers about is always called a "theory".

The point is: in science, sometimes theory means a well-supported explanation, and sometimes it really does mean "just a guess". That guess may be deep and mathematically rigorous, accurately replicating a lot of known properties of the universe, but there's no experimental or observational evidence (i.e. verified predictions indicating the hypothesis should supersede the status quo). If respected scientists don't believe that theory means well-supported, then why should anyone else? Why should laypeople accept that their definition is wrong, when scientists get to say that the word means what they want it to mean (neither more nor less)?

Is this discrepancy strictly confined to Physics, or do other branches of science also use theory this way?

We probably can't force physicists to change their usage, so should we chastise the educators and tell them to discuss these exceptions? Currently, they are lying to the public.

Edit #2: This question is about inconsistent use of terminology. If one group of scientists say that planet means "gravitationally-rounded body that has cleared its solar orbit" while another group says it means "wandering light in the sky", then that disagreement would impede the flow of knowledge. Hence, astronomers spent a great deal of effort to choose a single definition for use in scientific discussion. (Whether they chose the right definition or not is a separate issue.)

Accurate communication between scientists requires agreement about the definitions they share (laypeople may continue using whatever definitions they want). There is an unaddressed disagreement among scientists about the meaning of the word theory, which I have documented with references. Hence, I'm seeking to explore viewpoints about word usage among scientists.

  • 2
    This isn't really an answer, more of just some thoughts. It's not uncommon for different groups of people with different interests to use the same terms differently. In formal logic, a "theory" is literally any set of sentences in some formal language. I think philosophers often use "theory" in the same way. It probably depends on context, and it never hurts to ask "what do you mean by X?" to avoid misunderstanding. Without more context, it's a bit extreme to call what the educators are doing as "lying". Oct 29, 2019 at 20:09
  • 2
    What exactly needs reconciling? Scientists and educators are interested in related but different concepts, both of which are legitimate and motivated by their respective fields. They happen to share a label, but that is usually disambiguated by context, and not much of a problem.
    – Conifold
    Oct 29, 2019 at 20:09
  • 2
    This question strikes me as polemics more than philosophy, but I'll try give an answer regardless. Just between you, me, and the wall, though, science does not need 'advocates.' Advocacy is a political posture, and adding politics to science is like adding pop-rocks to soda: it ruins the pop-rocks and the soda, and makes a big effing mess in the process... Oct 29, 2019 at 21:21
  • @TedWrigley ugh. That's not at all what I am talking about.
    – Foo Bar
    Oct 29, 2019 at 21:31
  • As you can see, the word "theory" has different meanings : "1.a plausible or scientifically acceptable general principle or body of principles offered to explain phenomena"; this is the main scientific usage: Quantum Th. "2. a hypothesis assumed for the sake of argument or investigation; an unproved assumption, a conjecture"; this is used when some scientific paper makes a provisional assumption to be tested. "3. a belief, policy"; this can be used outside science : I've my theory about the existence of martians. Oct 30, 2019 at 14:59

3 Answers 3


Here are some considerations that may be a first step toward an answer:

  1. Normativity -

It seems to me the biggest difference here is that the public educators you mention all have a normative concept built into their definition of 'theory.' The Berkeley page says "strongly supported", and the other two say "well-substantiated" and "well-supported", respectively.

It is quite reasonable, then, to suggest that the question of what makes a thing a theory and what makes a thing a good theory are two separate questions. What makes a thing a theory is that it--say--is a set of related propositions. What makes it a good theory is evidence, substantiation, etc.

  1. Pragmatic Concerns -

You outline well a plausible motivation for public educators to build the normative component in. In short, we don't want the wrong kind of science skepticism to be brewed among non-experts.

When people say that something is merely a theory, they usually mean that--because it is JUST a theory--it has no normative authority over our epistemic states. Thus, climate science deniers and their ilk may be banking on the strict definition of theory--indeed, if all a thing needs to be to count as a theory is a set of sentences, then the very fact that a theory issues in X conclusion does not by itself give you any reason to believe that X is true. But those who are taught basic science need to understand that the very fact that a well-substantiated theory issues in conclusion X is itself reason to believe that X is true. They need to understand this way more than they need to understand the philosophically correct definition of 'Theory.' Sometimes nuance can get in the way of the stated goal of an educator.

In other words, it is the role of a layman to assume that a scientific theory BY DEFINITION has normative force on our epistemic states.

  1. Should scientists, epistemologists, etc., take on the normative definition of 'Theory'?

Absolutely not! It is the job of scientists and philosophers to understand that a theory really is "merely" a theory until it has been substantiated. It is their job to assess which theory has the most substantiation and to further add to that substantiation as well.

In short: Where the layperson should just assume that the best scientific theories of their day have normative force on their epistemic state (i.e., they should believe in accordance with science*), by contrast the job of the scientist and philosopher is precisely to understand and assess how--and whether--this normative force is generated.

  • It's been a long time since Philosophy 101, so it's going to take me a while to parse some of that.
    – Foo Bar
    Oct 30, 2019 at 2:32
  • @FooBar Feel free to comment with any questions or objections and I'd be happy to update the answer. Oct 31, 2019 at 4:25

I believe this question misunderstands that nature of scientific investigation. A theory is nothing more than a claim about (or model of) the processes lying behind some observable event. We do not need evidence of any sort to formulate a theory; evidence only serves to convince others (and ourselves) that our theory is functional and sound.

Obviously, a working physicist (or any working scientist) is going to rely on theories that have a proven track-record of evidentiary support. That only makes sense: designing a spacecraft or computer or what-you-will using speculative insights is a recipe for disaster. But by that same token, an academic or research physicist (or any academic scientist) must engage in a certain amount of speculative insight in order to advance the cause of science. If such insights are good, they amass a body of evidence over time and are worked into our understanding of the universe; if they are not, they don't, and they disappear into the ever-growing list of failed theories.

Without thought experiments we would never have real experiments, and thus would never generate evidence for anything we didn't already know. That's the nature of this beast, so trying to claim that thought experiments are not 'theories' hamstrings the entire scientific project.

  • My original question must have been unclear, since multiple respondents misunderstood it. Do you see the distinctions in my edited question?
    – Foo Bar
    Nov 4, 2019 at 12:43
  • I understand the distinction you' retrying to make, but I don't see why you're trying to make it. Scientists don't have a problem here. They use the term 'theory' to cover all these cases, and when necessary add qualifiers: 'accepted theory,' 'unsupported theory,' 'failed theory'... Denialists deny the adjectives: they refuse to look at evidence and thus reduce all theories to the same empty cipher. Nov 4, 2019 at 14:09
  • When a scientist runs across a denialist, of course he's going to push down on stricter definitions. You know, when a cowboy calls a cow a 'doggie' (like in that old song), nobody blinks twice, because everybody knows that a cowboy knows what a cow is. But if some tom-fool stands up and demands that cows are actually dogs, people will put their foot down and say: "No, you can't call a 'cow' a 'dog'." This isn't inconsistent definition or usage; it's correcting people who refuse to use language properly. Nov 4, 2019 at 14:17

Your question contains lots of misconceptions about science and critical discussion in general.

First, scientific knowledge is created by noticing a problem with existing ideas, proposing solutions to that problem and criticising the solutions until only one is left. No theory is immune from people spotting problems with it and proposing a better theory. There is no such thing as support. Support plays no role in science at all. A theory is either true or false. And any theory could be overturned by a competitor that solves a problem it doesn't solve, see

Epistemic justification - 'turtles all the way down'?


and "The Fabric of Reality" and "The Beginning of Infinity" by David Deutsch.

By the same token, bad theories are not rejected because there is some quantity called support and they have a bit less of it than standard theories. In many cases, these theories don't rise to the level where they are even a candidate to solve the problem under consideration, so talking about evidence gives them too much credit:

Why aren't creationism and natural science on the same intellectual level?

It is extremely dangerous to science to try to ban or police criticism of ideas that are currently popular among scientists. Future progress is dependent on being able to replace current ideas. In addition, the idea that we should be pro-science is often used to advocate particular political ideas in the name of science. People on the right and the left both do this and it is always a mistake.

Your second misconception is that by stopping vocabulary from being used in a way you disapprove of you will stop ideas you disapprove of. You have this the wrong way around. Terms are a shorthand for ideas. So the only way to stop terms from being used in ways you dislike is to refute the ideas they represent.

Finally, all ideas are theories so the idea that evolution or whatever is a theory is relevant to assessing it is hogwash. The idea that I'm sitting on a chair right now is a theory. But people who advocate the ideas that scientific theories are supported invite this objection because no finite set of facts implies any theory and nobody has ever come up with a theory of support that stands up to critical scrutiny. So the suspicion is aroused that every idea that isn't accepted as an uncontroversial fact should be rejected. Getting out of this mess requires rejecting the idea of support root and branch and persuading your opponents to reject it too.

  • 1
    your answer has very little to do with the question I asked.
    – Foo Bar
    Nov 4, 2019 at 12:16
  • Your question is riddled with misconceptions that you should correct. Pointing out those misconceptions is on topic, your protestations to the contrary notwithstanding.
    – alanf
    Nov 4, 2019 at 12:49
  • My question is not at all about how scientific knowledge is created. My question is not at all about how theories are confirmed or rejected. My question is not at all about banning or policing criticism. My question is not at all about stopping ideas that I disapprove of, except for the idea that words should mean whatever their user wants them to mean, to which I say "purple monkey dishwasher".
    – Foo Bar
    Nov 4, 2019 at 13:06
  • Your question makes false assertions about how scientific knowledge is created and epistemology more generally, e.g. that it involves support. The question can't be addressed properly without pointing out that you're trying to make people enact bad ideas about science and epistemology.
    – alanf
    Nov 4, 2019 at 13:09

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .