It seems simple enough: The job of science is to observe, describe, and explain the natural world through hypothesis and experimentation. A scientist will say, "I think this explanation is the reason for this observation, and I propose this experiment to test it." But the statement doesn't begin to convey the job at hand. Theories, hypotheses, laws, the scientific method -- even facts themselves -- dangle from the natural sciences like so many tree branches. How do the various parts fit together?
Let's start with scientific theory. Cambridge University's Stephen Hawking describes the essence of a theory in his best-selling book A Brief History of Time: "A theory is a good theory if it satisfies two requirements: It must accurately describe a large class of observations on the basis of a model that contains only a few arbitrary elements, and it must make definite predictions about the results of future observations." Eugenie Scott, a physical anthropologist and executive director of the National Center for Science Education, puts it more succinctly: "A theory is a construct of facts and hypotheses that attempts to explain a natural phenomenon." So, theories are neither guesses nor hunches.
Examples of good theories include Charles Darwin's theory of evolution, which explains how populations of organisms change into diverse forms over time. Combined with Gregor Mendel's experiments with garden peas, Darwinian thought laid the foundation for modern genetics and heredity. Then came Albert Einstein's theory of relativity, which describes everything from the properties of solar systems to the infinitesimal whizzing of atoms. Like many scientific breakthroughs, Einstein's theory started as nothing more than mental noodling but eventually led to technologies such as transistors and lasers (and of course the atomic bomb).
To say "good theories" implies that there are not-so-good theories, which takes us back to hypotheses and testing. Some would-be theories may not be testable. That is why science rejects theories based on supernatural claims, such as intelligent design. Established theories can be challenged by new observations that don't fit the old mold, and can be disproved or modified if observations don't agree with predictions. A classic example is geocentricism, the theory that the Sun and its planets revolve around Earth, which was slowly -- and against much resistance -- replaced by Suncentered theories postulated by Copernicus, Galileo, and Kepler.
If scientific facts (certified, repeated observations) are part of theories (explanations of facts) and some theories don't hold water, this equation implies facts are suspect, too. Pounding the table while saying "That's a fact!" implies facts are equal to capital-T truth. But scientific facts can change over time. Helen Longino, a Stanford University professor who teaches the philosophy of science, warns, "Facts are more solid than theories, but they aren't beyond question." Newton's, Galileo's, and Kepler's gravitational facts were considered universal until Einstein came along and predicted that gravity could actually stretch or shrink distances. However, in physics especially, if a set of facts supporting a theory survives skeptical onslaughts from generations of scientists, the theory becomes universally accepted, as in the case of the laws of gravity, motion, or thermodynamics.
A good theory can be tested, and must be. The theory of evolution is considered sound because years of evidence from many disciplines show that Darwin's ideas stand the test of time. Facets of evolution can be probed by new hypotheses and experimentation, adding knowledge to the theory as a whole. Such questions have focused on evolution's pace, whether new species appear gradually or more rapidly. Another has centered on whether new traits arise randomly or are selected by natural causes, as Darwin originally thought. We don't yet talk about the law of evolution, because the theory is still being refined and polished.
Theories and facts don't exist in a vacuum -- they are parts of our social fabric, influenced by community behavior. "Science is not extracultural," says Katrina Karkazis, a medical anthropologist who also works at Stanford. She points out that the power of values and opinion can cause societies to accept a scientific theory unquestionably, rather than methodically testing its strength and weaknesses. "Without healthy skepticism, science becomes dogmatic," she adds. Ironically, our fascination with technology tends to give the impression that scientific results are more real than they seem (but that's another story entirely).
Theories and facts are themselves part of an evolutionary process, popping onto the scene only to be tested by the scientific method. The fittest survive for another day and another challenge. The weak (think of phrenology) are destined for the dustbin of history, curious artifacts discarded by a community of questioning skeptics.
Christopher Thomas Scott is a contributing writer for Edutopia.