What is a Theory?

I’ll be honest: this article has nothing to do with a question my son asked, but it has everything to do with a show I was watching with him.

My son watches PBS the way most other kids his age watch Nick, Jr. or the Disney Channel. This has nothing to do with his mom and dad being snooty liberal types, and everything to do with the fact that his mom and dad are too cheap to pay for cable. Instead we have rabbit ears and one of those digital signal converters, and PBS comes in over the airwaves.

Anyway, I’m watching Thomas Edison’s Secret Lab with my son. It’s an odd little science show, with robots and a hologram of Thomas Edison, but it tries to teach science. The episode we’re watching is about one of the characters making a youth potion because he resents being too big to ride on his favorite amusement park ride, and this lead to an explanation of cell division and telomeres and the role they seem to play in aging. Then, one of the characters tells him that this is “just a theory” and “hasn’t been proven”.

That threw a huge red flag up for me.


The word “theory” has multiple definitions, of course. After all, it’s an English word. Here they are:

  1. a coherent group of tested general propositions, commonly regarded as correct, that can be used as principles of explanation and prediction for a class of phenomena: Einstein’s theory of relativity. Synonyms: principle, law, doctrine.
  2. a proposed explanation whose status is still conjectural and subject to experimentation, in contrast to well-established propositions that are regarded as reporting matters of actual fact. Synonyms: idea, notion hypothesis, postulate. Antonyms: practice, verification, corroboration, substantiation.
  3. Mathematics. a body of principles, theorems, or the like, belonging to one subject: number theory.
  4. the branch of a science or art that deals with its principles or methods, as distinguished from its practice: music theory.
  5. a particular conception or view of something to be done or of the method of doing it; a system of rules or principles: conflicting theories of how children best learn to read.
  6. contemplation or speculation: the theory that there is life on other planets.
  7. guess or conjecture: My theory is that he never stops to think words have consequences.
  8. (Idiom) in theory, ideally; hypothetically:

The problem with the word “theory” is that when it is used in common speech, it generally is used to mean something like the second or fifth definition. Theories are seen s somehow unproven; they’re things that need testing, unlike laws.

In science, theory most commonly refers to the first definition. LiveScience.com gives a good explanation of what a scientific theory is:

The University of California, Berkley defines a theory as “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.”

Any scientific theory must be based on a careful and rational examination of the facts. Facts and theories are two different things. In the scientific method, there is a clear distinction between facts, which can be observed and/or measured, and theories, which are scientists’ explanations and interpretations of the facts.

An important part of scientific theory includes statements that have observational consequences. A good theory, like Newton’s theory of gravity, has unity, which means it consists of a limited number of problem-solving strategies that can be applied to a wide range of scientific circumstances. Another feature of a good theory is that it formed from a number of hypotheses that can be tested independently.

They are also distinct from scientific laws:

A law is a description of an observed phenomenon that hold true every time it is tested. It doesn’t explain why something is true; it just states that it is true. A theory, on the other hand, explains observations that are gathered during the scientific process. So, while law and theory are part of the scientific process, they are two very different aspects, according to the National Science Teachers Association.


A hypothesis is distinct from a theory and, in fact, supports the validity of a theory:

  1. a proposition, or set of propositions, set forth as an explanation for the occurrence of some specified group of phenomena, either asserted merely as a provisional conjecture to guide investigation (working hypothesis) or accepted as highly probable in the light of established facts.
  2. a proposition assumed as a premise in an argument.
  3. the antecedent of a conditional proposition.
  4. a mere assumption or guess.

Please Don’t Confuse Them

A hypothesis is a testable proposition, a statement of an attempt to explain how and why a specific phenomena occurs. A theory is a general explanation for how and why some group of related phenomena work, that makes testable predictions and is supported by multiple observations and experiments. Evolution is a theory. Germs causing disease is a theory. Gravity is a theory. The structure of the atom is a theory. Theories are, in the end, things that are true as far as they go. Newtonian Physics was a theory, and it still is. But it was superseded by General and Special Relativity, which is also a theory.

So don’t dismiss something, because it’s “just a theory”.