Most of us have particular phrases that set our teeth on edge. Whether it’s the trite “live, laugh, love”, the almost inevitably misogynistic “you’d be prettier if you smiled”, or the blatant lie only uttered by those addicted to high drama in all its form that is “I hate drama”, there are some stock soundbites guaranteed to irritate.
Few quips raise my blood pressure quite as much as “whatever happened to global warming?” which is seemingly only uttered during typically wet or cold weather and never during unseasonal heatwaves. One that does manage that feat, however, is “only a theory”.
This phrase betrays a common misconception about science. It frequently appears in debates about scientific concepts such as climate change or evolution, with the implication that a theory is something that is hedged, provisional, or yet to be proved. But what does it really mean when something is labelled “only a theory”?
Scientifically speaking, a theory is quite different from a guess or intuition. A scientific theory explains some aspect of the natural world, using evidence-based on experimentation, observation, or a similarly rigorous approach. To say that something is a theory is to emphasise its explanatory powers.
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Global warming is one frequent target of this approach, as is evolution. The theory of evolution by natural selection is often described by critics as “only a theory”. However, it is one of the most well-supported and widely accepted theories in biology. Countless experiments, observations, and peer-reviewed research have gone into producing the model that we now call the modern synthesis, and which underpins our understanding of genetics.
Charles Darwin was raised in the scientific culture of early 19th century England. Then the scientific ideals of Francis Bacon were venerated, prizing observation and inductive reasoning above all else, and dismissing speculation.
Inductive reasoning makes generalisations based on repeated observations. If you have witnessed dozens of sunrises to the east (and none elsewhere) you can make a reasonably safe inductive inference that the sun will rise in the east tomorrow. Darwin knew that his mechanism of natural selection would not qualify as observable in the same way as a sunrise, so his great work On the Origin of Species gives an exhaustive account of the observations that underpinned his reasoning.
Before a scientific idea becomes an accepted theory, it usually starts as a hypothesis. A hypothesis is an idea that can be tested, an educated guess that serves as a starting point for scientific investigation. Hypotheses are a means by which scientists can frame a question and design experiments to gather evidence that will prove or disprove it.
Hypotheses are formulated based on existing knowledge and observations, and they serve as a guide for researchers as they seek to answer specific questions. A well-constructed hypothesis is both falsifiable and specific; it must be able to be proven wrong through experimentation and clearly define the variables that are to be studied.
A doctor might notice that patients eating a particular food are less likely to get a specific disease and formulate a hypothesis such as “patients who consume greater quantities of food X will have lower rates of disease Y”. Using controlled experiments, this hypothesis can be tested so that results will either support or refute it.
In the scientific process, hypotheses are refined and modified as new evidence emerges. When a hypothesis consistently withstands rigorous testing and is supported by a substantial body of evidence, it may evolve into theory. So, when you encounter the phrase “only a theory”, remember that it signifies a deep, well-founded understanding of the natural world.
Part of the confusion arises from the use of theory to describe hypotheses. In the eighteenth century, phlogiston was the prevailing explanation for combustion. According to this hypothesis substances contained a mysterious substance called phlogiston, released during combustion and believed to be the reason it could burn.
This hypothesis could explain some observations. When wood burned, it appeared to lose weight, evidence phlogiston was being released. A turning point was the discovery of oxygen by Antoine Lavoisier in 1778. Lavoisier, who would shortly after identify hydrogen, demonstrated that combustion was not the release of phlogiston but rather the combination of a substance with oxygen. This discovery revolutionised the understanding of combustion and chemical reactions and was instrumental in the growth of modern chemistry.
Phlogiston shows how scientific ideas are always subject to revision when more evidence is presented. Scientific theories represent our most robust and well-supported explanations of the natural world. Hypotheses are the starting points for scientific inquiry, guiding researchers as they seek answers to specific questions.
- Stuart Mathieson is a postdoctoral fellow in the School of History and Geography at Dublin City University