(This piece is slightly modified from one originally published on the Adventist Today web page – apologies to those who have already ready it in other fora. More brand new words tomorrow!)
“Evolution is a fact” is something we tend to hear from one
side in debates about origins. I’d argue that this statement reflects a
misunderstanding of the roles of facts, theories and laws in science.
At the same time, from the other side of those same debates,
we tend to hear “evolution is (just) a theory”, which is equally unfortunate as
a way of thinking about what ‘theory’ means in science.
I want to, as clearly as I can, briefly outline the meanings
in science of the terms ‘fact’, ‘theory’ and ‘law’, and to explain why a
theory, no matter how well supported by evidence, never turns into a fact.
Evolution is, in some circles, a controversial theory, and
therefore a bit awkward to use as an example for this discussion, since it
brings in strong emotions and strongly held views on the part of readers. (And,
just quietly, I’m a physics guy, not a biology guy.) So, instead, I’ll use the
example of gravity.
Here is a fact about gravity: close to the surface of the
earth, if any object (that has mass) is unsupported, it will accelerate toward
the centre of the earth with an acceleration of about 32 feet per second per
second, or about 9.8 metres per second per second. If you have an object and a
stopwatch handy right now (and your phone probably has a stopwatch function),
and have done a little high school physics, you can test this fact.
In science, ‘fact’ is used to refer to a single piece of
data, the result of a measurement. Other facts about gravity include the fact
that it decreases in strength as we move away from the centre of the earth, and
that every object that has mass exerts a gravitational force on every other
object that has mass. With sophisticated-enough instrumentation, all these
facts can be measured and expressed in numbers.
Not all of science is physics, though, as my students often
remind me. It’s a fact in chemistry that sodium chloride (table salt) has a
cubic lattice structure between its atoms, and a fact in biology that living
things contain DNA, and a fact in geology that most rocks contain a lot of
Let’s leave ‘theory’ on the side of our plate for the
moment, because it’s the most complicated, and talk about ‘law’. In science, a
law is a mathematical relationship between quantities. The most famous law in
science is probably Einstein’s E = mc2, which describes the relationship
between matter and energy.
The key law in gravity, which was formulated by Isaac Newton
(and I do apologise for those who find equations challenging!), is F =
(Gm1m2)/r2 In words, it says that if there are two masses, m1 and m2, a
distance r apart, the force F between them is given by this law, where G is
called the ‘universal gravitational constant’.
A law is powerful because it makes a relationship clearer. A
couple of paragraphs ago I said that the force decreases with distance, but the
law gives more detail. It shows that the force decreases with the square of
distance: if the objects are 2 times as far apart, the force is only ¼ as
A theory is a human mental creation that explains facts and has withstood the test of experiment. This view of the nature of science and of theory is owed to philosopher of science Karl Popper. A theory in science has descriptive, predictive and explanatory power. That is, a theory describes the world as we see it and experience it. It allows us to reliably predict how the world will behave in future in a particular set of circumstances, and it explains why the world is as it is. If we make a prediction using a theory, and then conduct the experiment and the prediction fails – the world does not behave as the theory leads us to expect – then Popper would say the theory has been ‘falsified’ and should be discarded. The theories that make up science at any given moment are the ones that have been tested many times and have never been falsified. Einstein neatly summed up Popper’s perspective: “No amount of experimentation can ever prove me right; a single experiment can prove me wrong”.
A theory is not held to be ‘true’ in any final sense under
this view: at best, it is the most powerful theory available, that explains the
greatest number of facts, and has not been falsified. A new experiment may yet
be conducted that will falsify it, and if that occurs the theory will need to
be discarded and replaced with a better one.
The first – and longest-lived – theory used to explain our
experience of gravity was proposed by Aristotle. He suggested that things in
the universe have their ‘natural station’, the place where they belong. Things
mostly belong on the ground – even birds – so when we lift them above the
ground they are being lifted out of their natural state, and if they are not
prevented from doing so, they will return to it. If we lift a book from the
floor to a table, it is out of its natural place, and if the table were not
there to prevent it doing so, the book would return to its natural place on the
Aristotle’s theory of gravitation applied only on Earth,
since he also believed that the heavens were a different domain from Earth,
with different rules and processes. The contribution of the next great theorist
of gravity, Isaac Newton, was to apply the same rule to objects in space like
the moon and the planets that was used to explain how things move on Earth.
Johannes Kepler had created rules that described the motion
of the planets in purely mathematical terms, but did not explain why the
planets moved the way they did. Most times laws are derived from theories, but
we could argue that Kepler’s laws were not drawn from a theory. They had
descriptive and predictive power – Kepler could tell you when the next eclipse
would come – but not explanatory power. Newton developed the theory that any
two objects with mass exert a force on each other, and – crucially – that this
is true in the heavens as well as on Earth. The same force that caused his
(probably apocryphal) apple to fall from the tree to the ground explained the
motions of the heavenly bodies. Newton had a theory, not just a law, because in
addition to description and prediction, it was capable of explanation. Newton’s
theory of gravity still works well for everything we encounter in everyday
life, but for much more extreme environments, such as near the event horizon of
a black hole, it breaks down. For those specialised contexts, it has been
replaced by the theory of General Relativity proposed by Albert Einstein. The
mathematics gets very complex very quickly, but in words, Einstein’s theory can
be stated as ‘matter tells space how to curve, space tells matter how to move’.
Gravity is explained, not as a force between objects, but as mass causing
curvature in the local space, which then causes mass to move differently.
Einstein’s theory is considered ‘better’ than Newton’s
because it is more universal – it can be applied everywhere in the universe,
whereas Newton’s theory breaks down in some situations.
All three of the theories described – Aristotle’s, Newton’s
and Einstein’s – explain the fact that a dropped book or apple will fall toward
the ground. Aristotle’s does not have an associated law – a mathematical
statement about how rapidly the apple will fall – while Newton’s theory does
include a law. Einstein’s theory also includes laws, but the mathematics are
too complex to go into here.
I hope these examples have helped to explain why a theory
can never turn into a fact or a law, no matter how much evidence it has behind
it. These are three different things with different qualities, each important
I suspect that when someone says “evolution is a fact”, they
are using the word ‘fact’, not as a scientist would, but in the everyday sense
of ‘not fiction’. They mean that a textbook on evolutionary theory in the
library would not be placed with the novels and short stories, but with the
other books that contain true information about the world. It’s probably still
an unfortunate usage, though, since the claim being made falls within science,
so the language used ought to be the careful language of science.
Evolution is a theory. It is one that explains, not one
fact, but an enormous variety of facts about the diversity and the
characteristics of life on Earth. It is a theory that was proposed more than
150 years ago, and it has been tested in a wide variety of ways. The
fundamental concepts have not been falsified, but significant elements have
been changed and updated. Darwin did not know about genes when he wrote ‘On The
Origin Of Species’, for example, or DNA. He did not have access to the vast
array of data about living things that modern scientists can draw on. The
modern evolutionary synthesis includes the recognition that gene transfer plays
a much greater role than previously thought, for example, so that less of the
‘heavy lifting’ of generating new characteristics must be borne by mutations.
When people say “evolution is (just) a theory”, they are
drawing on the common, everyday use of the word, rather than the scientific
use. We say “I have a theory” when we mean a guess, a hunch, an untested
brainwave. Saying “evolution is a theory” is thought of as a way of saying that
it is unsupported, held without evidence, untested. These same people would
tend not to say “gravity is (just) a theory”, although in scientific terms,
gravity is indeed a theory. Or, at least, there are multiple theories of
gravity that explain the facts of gravity, some of which provide mathematical
laws, and Einstein’s is currently the best theory we have.
Theories do change, and Einstein’s theory may well be
replaced by an even more powerful one in the future. There are interesting
problems at the boundaries between General Relativity and quantum theory, for
example, that may revolutionise our understanding in the future. But a book
will still fall from a table, and an apple from a tree. A change to the theory
of gravity will not enable us to suddenly, unaided by technology, safely walk
off the roof of a tall building and just float. The facts of gravity will
remain the same if the theory used to explain them changes.
The same is true of evolutionary theory. It has changed in
the past and is likely to continue to change. Alternative candidate theories,
including special creation and intelligent design, already exist, and already
claim to explain the same facts. When the theory changes, the facts will not
change. The DNA that forms the genetic blueprint for a jellyfish will not
suddenly begin to produce a lion instead.
It’s probably a discussion for another article, but to date
the alternative candidate theories – special creation and intelligent design –
have not demonstrated descriptive, predictive and explanatory power in the same
ways and to the same extent as the modern evolutionary synthesis. There are
efforts to make these demonstrations, which will continue to be tested against
the facts of biology.
I hope this brief discussion has been helped you to understand the meanings of the terms ‘fact’, ‘theory’ and ‘law’ in science, and to be aware when these terms are being used confusingly in discussions around origins as well as other scientific topics such as vaccination, genetic modification and climate change. If we can communicate clearly and accurately, and go forward together in good faith, we have more chance of finding our common ground.
For ease of navigation I will include links to each of the other posts in this series at the bottom of each post.
Why I think it’s important to understand evolution
Cosmogenesis, abiogenesis and evolution
Evolution and entropy
Radiometric dating and deep time
Four Forces of the Universe
Probability and evolution
Species and ‘baramin’, macro- and micro-evolution
Mitochondrial Eve and Y-chromosomal Adam
Complexity – irreducible and otherwise