Scientific theories

Theories and mere theories

In everyday language, the term ‘theory’ is often used dismissively. When people say that an explanation or account is “only a theory” they are likely to mean that it is untested, tentative, perhaps a bit vague.

We also use the term to describe a grand view about the nature and meaning of life. Mohammed bin Rashid Al Maktoum, Prime Minister of the United Arab Emirates, uses ‘theory’ in this latter sense when he says that his “theory on life is that life is beautiful. Life doesn’t change. You have a day, and a night, and a month, and a year. We … can be miserable or we can be happy. It’s what you make of your life”.

In science, by contrast, “theory” is a technical term used to describe a comprehensive account of some aspect of nature, supported by a large body of converging evidence, based on repeated observations and testing, usually integrating and generalising hypotheses, and making consistently accurate predictions across a broad area of scientific inquiry.

When critics dismiss the theory of evolution because it is “only a theory” they are using the term ‘theory’ equivocally (see the Learning to Spot Fallacies document from week one).

When used to describe the scientific view that processes such as mutation, natural selection, and genetic drift lead to change in the gene pool of a population from generation to generation, the theory of evolution is a scientific theory: a comprehensive account of a vast number of observations (many more now than the theory’s originator Charles Darwin had to hand) repeatedly tested and which allows scientists to accurately predict that they would, for instance, find fossils intermediate between fish and limbed terrestrial animals in sediments that were about 375 million years old.

Theories allow scientists to move beyond particular observations and hypotheses, to make broader predictions about natural events or phenomena that have not yet been observed. Newton’s theory of gravitation, for instance, allowed scientists to predict the existence of the planet Neptune based on observations of the movements of Uranus that could not be explained by the known planets.

It does not follow that theories cannot be changed. Just as scientists must remain open to the possibilities that their hypotheses might be falsified by an experiment or observation, so they must be open to the possibility that theories will prove to be mistaken, either in points of detail or more generally.

In the 16th and 17th Centuries, for instance, the Ptolemaic theory which had the Earth at the centre of the universe was replaced by the heliocentric theories of Copernicus, Galileo, and Kepler, and in the 19th Century, the theory that diseases such as cholera and the Black Death were caused by “bad air” (the Miasma Theory) fell to the Germ Theory – the theory that some diseases are caused by micro-organisms.

The Ptolemaic and Miasma theories were supported by observations and had some explanatory power, but both were overtaken by developments (the invention of telescopes and microscopes for instance) that made more precise observations possible, and by theories that explained a larger and broader range of observations.

Because theories are supported by repeated observations and survive repeated testing and explain a broad range of observations and patterns, however, well-established theories are less likely to be overthrown than isolated hypotheses.

So although all theories are subject to revision, some are so well established by observation and predictive power that they’re unlikely to be abandoned: it is unlikely we will reject the view that the Earth orbits the Sun, or that germs cause at least some diseases: the heliocentric theory and germ theory are theories, but not mere theories.