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Why is it so hard to understand what a theory is?

Even the dictionary doesn’t get the definition right.
Credit: Ulia Koltyrina / Adobe Stock
Key Takeaways
  • There is widespread confusion about the word “theory.”
  • The confusion comes from its usage in scientific versus more colloquial contexts.
  • Interest groups with misguided intentions take advantage of this confusion to undermine the value of scientific work.

There is widespread confusion about the word theory. A lot of people interpret the word to mean undetermined knowledge based mostly on speculative thinking. As in, “Oh, that’s just a theory.” The word is used both to indicate things we do know — based on solid empirical evidence — and things that live in the speculative realm, that we cannot be sure about. 

The same word thus has two conflicting meanings. That is not a good mix, especially when certain scientific theories speak directly to people’s religious and value-based sensitivities, such as the theory of evolution or the Big Bang theory. The definitional confusion helps feed the very real danger of being duped by groups with specific agendas that want to manipulate established scientific knowledge to confuse people.

Even the dictionary is confused

Looking at the word’s listing in the New Oxford American Dictionary does not help:


  • a supposition or a system of ideas intended to explain something, especially one based on general principles independent of the thing to be explained: Darwin’s theory of evolution.
  • A set of principles on which the practice of an activity is based: a theory of education.
  • An idea used to account for a situation or justify a course of action: my theory would be that…

So, there is usage within a scientific context (“the theory of …”) and in a subjective context (“my theory is…”). This creates an obvious problem.

When used in the context of a phrase, as “in theory,” it gets worse still. According to New Oxford, “used in describing what is supposed to happen or be possible, usually with the implication that it does not in fact happen.” [My italics.] In this context, “in theory” means something that is probably wrong. 

No wonder there is confusion. It is confusing!

To clarify the meaning of theory, it is essential to understand the context in which the word is being used, and to keep different contexts separate from each other. So, if a scientist is using the word theory, as in “theory of relativity,” “theory of evolution,” or “Big Bang theory,” it should be understood as a statement within a scientific context. In this case, a theory is certainly not mere subjective speculation, nor is it something that is probably wrong. Quite the contrary, it is something that has been scrutinized by the scientific process of empirical validation and has, so far, passed the test of explaining the data. 

Words matter in science

Unfortunately, even within the scientific context the word is misused, which only adds to the confusion. For example, “superstring theory” refers to a speculative idea in high-energy physics where the fundamental building blocks of matter are not elementary particles, but tiny, vibrating tubes of energy. Given the lack of empirical support so far for the idea, “superstring hypothesis” would be a more appropriate characterization. Scientists may know that superstring theory remains hypothetical, but most people will not know that. We should be more careful qualifying our statements, especially if we want people to understand the difference between, say, the theory of relativity and superstring theory.

A scientific theory is an accumulated body of knowledge constructed to describe specific natural phenomena, such as the force of gravity or biodiversity, that has been vetted by the scientific community. An accepted theory conveys the best that we can come up with at a given time to make sense of certain kinds of natural phenomena — for example, how atoms work, or how bacteria reproduce.

As our understanding of natural phenomena changes, theories can change as well. This does not have to mean that the old theories are wrong, although they could be. (As an example, take the presumed existence of the ether as a medium to support the propagation of light waves in 19th century electromagnetic theory.) Theories change, or evolve, because older versions of them have a limited range of validity that cannot cover newly discovered phenomena. Newton’s theory of gravity works really well to send rocket ships to Neptune, for example, but it is not useful to describe a black hole. New theories are born from cracks in old ones. Progress in science happens when ideas, and the theories that they spawn, fail.

The misplaced suspicion placed on certain scientific theories comes from confusing subjective speculation with objective description. A scientific theory is different from a scientific hypothesis. A hypothesis is an idea not yet empirically tested, and hence still not vetted by the scientific community. Scientific theories are based on hypotheses that have been tested and vetted. Much confusion could be avoided if the word theory were used only in the correct context. The strategy of exploiting its double meaning to confuse or misguide popular opinion should only fool those who don’t know or choose to neglect what the word theory means when it is used in science.


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