A hypothesis is a specific prediction you set out to test; a scientific theory is a broad explanation that has already survived a great deal of testing. My hypothesis is that the plants under the blue filter will grow faster. The germ theory of disease explains why sterilising instruments reduces infection. A hypothesis is the starting point; a theory is a well-supported endpoint.
The core difference
Rank them by how much evidence stands behind them. A hypothesis is a proposed answer, narrow enough to be checked, and not yet confirmed. A theory, in science, is an explanatory framework that has been tested repeatedly, has survived attempts to disprove it, and now organises a large body of observations.
A theory is not a hypothesis that got promoted for good behaviour. They do different jobs. A hypothesis predicts one outcome; a theory explains why a whole class of outcomes happens.
How to use each
- Our hypothesis was that shorter shifts would reduce error rates on the ward.
- Three of the four hypotheses were not supported by the data.
- Plate tectonics is the theory that explains earthquakes, volcanoes and the fit of the continents.
- The theory makes predictions that can be tested, which is what keeps it scientific.
Note the plural: hypotheses, pronounced hy-POTH-uh-seez. Hypothesises is not a plural noun, though hypothesises does exist as a verb form ("she hypothesises that...").
The test you can run
Ask: could a single well-designed study settle this? If yes, it is a hypothesis. If it would take a field's worth of studies across decades to establish, and it explains many separate findings at once, it is a theory.
Second check: a hypothesis can usually be written as a sentence in the form "if X, then Y". A theory cannot be squeezed into that shape, because it is a system of ideas rather than a single prediction.
Be honest about everyday usage
In ordinary speech, theory means a hunch: "my theory is he never posted the form." That usage is perfectly good English and centuries old, and there is nothing wrong with it at the pub. The trouble is that it collides with the technical sense, which is how the line "it's only a theory" ends up being used to dismiss well-evidenced science. The two senses are genuinely different, and the everyday one is not what a scientist means.
If you are writing where both readings are possible, avoid the ambiguity: say hunch, guess, suspicion or working assumption for the informal sense, and reserve theory for the technical one.
Two neighbours worth knowing
A law is not a theory that has been proved. Laws describe what reliably happens (often as an equation); theories explain why. Neither outranks the other.
A model is a simplified representation used to work with a theory — it can be wrong in its details and still useful. Calling something a model rather than a theory is often the more accurate and more modest choice.
Frequently Asked Questions
Is a theory just a guess?
Not in science. There, a theory is an explanation supported by a large body of tested evidence. In everyday speech <em>theory</em> does mean a guess, which is why the word causes so much confusion — if you mean a guess, say hunch or suspicion.
Can a hypothesis become a theory?
Not directly. A confirmed hypothesis becomes a supported finding. Theories are built when many findings need one explanation that ties them together, so a theory is a different kind of statement rather than an upgraded hypothesis.
What is the plural of hypothesis?
<em>Hypotheses</em>, pronounced hy-POTH-uh-seez. It follows the Greek pattern, like crisis/crises and analysis/analyses.