One often hears the naive critic of science say "it's only a
theory", meaning that it is "unproven" and once
"proven" will become a law or a fact. This is not surprising, because
such a notion is consistent with what is sometimes erroneously taught in Junior
High General Science classes. The statement is, nonetheless, completely false
and demonstrates two common misunderstandings about science.
The first misunderstanding is that science "proves" theories. Theories are never "proven" in the absolute sense of mathematics. (Indeed, even mathematical theorems are not proven absolutely, but that's another discussion.) Theories are only confirmed by observation; but such confirmation is always tentative. No matter how well or how long a scientific theory has been confirmed, it is always subject to falsification or correction by new observations.
Consider the Theory of Newtonian Mechanics. For 200 years, it was the uncontested standard of scientific "truth". Its success was so total that some philosophers (Kant) asserted it was not just an empirically confirmed theory, but an "a priori truth". We now know better. At very high speeds and at very small distances, the Theory of Newtonian Mechanics has been conclusively falsified. It has been replaced by the combined Theories of Relativity and Quantum Mechanics. Yet, Newtonian Mechanics remains an excellent approximation to these more correct theories under what might be called normal conditions - thus, even when falsified, a scientific theory may remain useful.
The second misunderstanding is that a theory is an "unproven" law or fact, or, in light of the preceding discussion, an unconfirmed law or fact. Scientific theories, laws and facts are three very different types of statements. A few historical examples will illustrate the differences.
Boyle's Law (V = kT/P) relates three facts about a gas - Volume, Temperature, and Pressure - and is familiar to all SCUBA divers. The Kinetic Theory of Gases (Maxwell and Boltzmann) not only explains why Boyle's Law is approximately true, but also why and when it is demonstrably false.
The Law of Fixed Proportions states that chemical elements combine with each other in simple integer ratios by weight. It is based upon such facts as: carbon dioxide is made from 3 parts carbon and 8 parts oxygen. The Atomic Theory of Matter (Dalton) attempted to explain why this is true. The Quantum Mechanical Theory of Atomic Structure (Bohr, Heisenberg, Pauli, Pauling, et al) explains why it is only approximately true, and also why and when it is demonstrably false.
Mendel's Laws state that inherited characteristics are propagated to subsequent generations in frequencies expressible as certain simple integer ratios. It is based upon various facts Mendel observed in growing peas. The Theory of Molecular Genetics (Sutton, Morgan, Dobzhansky, McClintock, Watson, Crick, et al) not only explains why Mendel's Laws are approximately true, but also why and when they are demonstrably false.
Hubbell's Law states that the speed at which a distant galaxy is receding from Earth is directly proportional to its distance from Earth. It is based upon certain facts, including the red shifts in spectrographic analysis of light from a certain type of star. Big Bang Theory (Friedmann, LeMaitre, Gamov, et al), based upon The Theory of General Relativity (Einstein), explains why Hubbell's law is approximately true.
Before attempting any explicit definitions, let's return to that old Junior High fallacy which states that: "Hypothesis becomes theory becomes law, as degree of proof increases". A hypothesis is indeed an idea requiring further research. When sufficiently confirmed, a hypothesis may become a theory, a law, or a fact. "A fact", one might ask? "Aren't facts so certain they require no confirmation?" This is another common misconception. A very brief history of Newtonian Mechanics will hopefully illustrate the nature of scientific facts.
Galileo, arguably the first modern scientist, supposedly dropped balls from the Leaning Tower of Pisa. It probably did not happen, but Galileo did experiment with rolling balls on inclined planes and did formulate perhaps the first rigorous laws for kinetics and freefall motion (s = 16t^2). He made various observations - a ball rolling down a 30 degree inclined plane travels X inches after one second, 4X inches after two, 9X inches after three ... - which became the facts summarized by his laws. These precise observations are how science replaces everyday facts, such as "things fall when dropped", with scientific facts. But some everyday facts, such as "heavy objects like rocks fall faster than light ones like feathers", must be rejected when scientifically examined. Notice also that Galileo's facts depended upon a new concept (or theory?) of time as a regular and measurable parameter (Galileo supposedly used his pulse).
Newton generalized Galileo's work with his Theory of Mechanics (including the law F = ma) and his Law of Universal Gravitation (F = GMm/r^2), both presented in his seminal work, "Mathematical Principles of Natural Philosophy". These subsumed Galileo's Laws as approximate special cases. Newton's Law of Gravitation was based upon a mysterious attractive force between any two masses. The force is "mysterious" because it was, at least to Newton. He tried but never did succeed in formulating a theory of gravity, which would explain "why" his universal law was true or "how" it worked. Using his Law of Gravitation and his Theory of Mechanics, Newton was able to explain numerous facts (the motion of the planets in the sky, the movement of the tides, etc.) and laws (Kepler's and Galileo's).
Einstein did develop a theory of gravity, called General Relativity, which does explain "how" gravity works and "why" Newton's Law of Universal Gravitation is approximately true. The Theory of Relativity (Special and General) also predicted that Newton's Theory of Mechanics would be wrong at very high energies - that measurements of mass, time and distance would vary depending upon the velocity of the observer. This prediction has been verified in millions of experimental observations in particle accelerators. But under normal conditions - those encountered in everyday life - Newtonian Mechanics is an excellent approximation to these more correct theories; and it remains the basis of the science underlying most engineering applications.
Notice how the terms "fact", "law" and "theory" are used in the above examples. This is consistent with the way they are generally used and understood in science. Scientific facts, laws and theories are three very different types of statements. One sometimes hears the word "theory" used in place of the word "hypothesis" - as in "I have this theory that ..." - but this is an abuse of the word, possibly motivated to avoid the pretentious sounding word "hypothesis". If formal definitions of the terms are requested, one might offer:
A scientific fact is a controlled, repeatable and/or rigorously verified observation.
A scientific law is a statement of an observed regularity among facts, often expressible as a simple mathematical relationship.
A scientific theory is an integrated conceptual framework for reasoning about a class of phenomena, which is able to coordinate existing facts and laws and sometimes provide predictions of new ones.
Theories often explain "why" laws and facts are "true" or "how they work". In the above discussion, notice that theories often have multiple names associated with them. This is not surprising, because theories are much more complex. Notice that not only theories and laws, but also facts may be falsified by new observations. The pre-Galileo "fact" that heavy objects fall faster than light ones was falsified. The Newtonian "fact" that mass, time and distance do not vary with velocity was falsified. The chemical "fact" that there are only three elemental forms of carbon - diamond, coal and graphite - was falsified.
This last falsified fact is another example of the interplay between hypothesis, fact and theory. Consider those Rice University chemists who formed the "hypothesis" that there was another elemental form of carbon in which the atoms would "wrap" into spherical "soccer balls". Quantum Mechanical Theory seemed to say it could happen. So they experimented, and they discovered Carbon-60, "BuckminsterFullerine". It's not a theory or a law. It's a Fact! Yet, it's a fact that can only be stated and understood in terms of the Atomic Theory of Matter. This is also not unusual. Many scientific facts are "theory-laden", meaning they can only be stated in terms provided by a scientific theory. The spectrographic facts supporting Hubbell's Law are similarly "theory-laden".
In normal usage, the word "Evolution" often refers to both a fact and a theory. Evolution is a fact. It has been directly observed. Evolution is the change over time of the distribution of genetic alleles ("genes") in a population. In that sense, it is populations that evolve and not individuals. Evolution has been observed in numerous situations. A population of a single species - individuals all able to mate and reproduce - may be separated and subjected to different environments. Over time, the distribution of alleles in the two populations will diverge. At some point, individuals from the two populations, all descended from the same original population, are no longer able to mate and reproduce. A single species has evolved into two species. Evolution is an observed fact.
Darwin's Theory of Natural Selection, augmented by the Theory of Molecular Genetics, explains the fact of evolution, as well as the facts of the fossil record and much else in modern biology. Notice the word "augmented". The later theory did not falsify Darwin's concept of natural selection, but rather enriched and expanded it, providing explanations for phenomena that Darwin had merely observed. This is also not unusual in the development of a science. The Classical Theories of Heat, Energy and Thermodynamics similarly augmented The Theory of Newtonian Mechanics without falsifying it.
The word "Evolution" is sometimes used to refer to the combination of the fact of evolution, the two above mentioned theories and the hypothesis that all life on Earth has evolved from a common ancestor. Properly speaking, this last is the hypothesis of abiogenesis. It is, indeed, lacking confirmation, although most biologist do think it is true. (An interesting historical aside is that Darwin never once used the word "evolution" in the original "The Origin of Species".) In either usage, Evolution is essential to modern biology. It has been said that nothing in biology makes sense except in light of Evolution.
ŠJames S. Freeman, 2002