Evolution is not "only a theory"

People who try to discredit the theory of evolution, preferring instead to rely on the Book of Genesis, will often say: “But it’s only a theory”. The implication is that it is unreliable and unproven and thus no more worthy of trust than the creationist view that they espouse.

However, what they are doing is misunderstanding what the word “theory” means. To them, something that is a theory is, by definition, “theoretical”. It is a back of the envelope idea, something thought up as an explanation that is no more than a thought and without any firm evidence to back it up. Another word meaning the same thing would be “hypothesis”.

That is indeed one meaning of “theory” but it is not the only meaning. To a scientist, a theory is a set of ideas that constitutes an explanation of how something works. It is the next stage beyond the hypothesis because it incorporates the evidence that any reasonable person would accept. It is the generally accepted account of the phenomenon in question, incorporating the laws and principles that govern what is known and observed.

There are many theories of the latter kind. The theory of gravity is one such, in that it is an explanation of how large masses exert attractive forces on smaller masses. Another is the heliocentric theory that describes how the sun sits at the centre of the solar system and the planets move round it in their orbits.

Neither of these theories can be described as unproven hypotheses, although this might well have been the case some centuries ago. When Galileo advanced the heliocentric theory of Copernicus he was thought to be a dangerous radical whose thinking was disputed because it challenged the teaching of the Church.

The theory of evolution has also been challenged because it is thought in some circles to be contrary to religious teaching. The “only a theory” jibe is therefore thrown at it just as it was at Galileo’s theory.

However, just as the heliocentric theory has moved from the “hypothesis” meaning of the word to that of “accepted explanation”, so has the theory of evolution. The evidence for evolution is so overwhelming that it has long moved beyond being a hypothesis, although this was not the case when Charles Darwin first proposed it. It is now a theory that is accepted as fact in the same way that the heliocentric theory or the theory of gravity is accepted as fact.

To say “only a theory” implies that there is something better than a theory, and that if evolution was worth its salt it would be this something else. However, outside the world of mathematics there is nothing better. Absolute proof of anything is impossible unless the terms are defined so rigorously as to be make proof inevitable, as happens with mathematical statements and formulae. In the real world there are plenty of accepted facts, but they are encased in what are conventionally called theories. They are proven for all practical purposes and are therefore completely reliable. There is nothing more factual and reliable that a well-formulated theory, in the non-hypothesis sense, and evolution is one such.

© John Welford