Scientific observations are the fuel which power scientific discoveries and scientific theories are the engine. Theories allow scientists to organize and understand earlier observations, then predict and create future observations. Scientific theories all have common characteristics which differentiate them from unscientific ideas like faith and pseudoscience. Scientific theories must be: consistent, parsimonious, correctable, empirically testable/verifiable, useful, and progressive.
1. What Is a Scientific Theory?
Scientists don't use the term "theory" in the same way that it's used in the vernacular. In most contexts, a theory is a vague and fuzzy idea about how things work — one with a low probability of being true. This is the origin of complaints that something in science is "only a theory" and so isn't credible. For scientists, a theory is a conceptual structure used to explain existing facts and predict new ones. According to Robert Root-Bernstein in his essay, "On Defining a Scientific Theory: Creationism Considered," to be considered a scientific theory by most scientists and philosophers of science, a theory must meet most, if not all, of certain logical, empirical, sociological and historical criteria.
2. Logical Criteria of Scientific Theories
A scientific theory must be:
- a simple unifying idea that doesn't include anything unnecessary (Occam's Razor)
- logically consistent (contradictions aren't allowed)
- logically falsifiable (there must be possible or theoretical situations in which the theory would be invalid)
- limited, so it's clear whether data verifies, falsifies, or is irrelevant (i.e., it doesn't presume to explain absolutely everything)
The logical criteria are cited in discussions about the nature of scientific theories and how science differs from nonscience or pseudoscience. If a theory includes unnecessary ideas or is inconsistent, it can't really explain anything. Without falsifiability, it is impossible to tell if it is true or not, so we correct it via experimentation.
3. Empirical Criteria of Scientific Theories
A scientific theory must:
- be empirically testable or lead to testable predictions or retrodictions (use present information or ideas to infer or explain a past event or state of affairs)
- make verified predictions and/or retrodictions
- lead to reproducible results so others can double-check
- include criteria for determine whether data is factual, artifactual, anomalous or irrelevant
A scientific theory must help us understand the nature of our data. Some data may be factual (verify the theory's predictions or retrodictions); some may be artifactual (result of secondary or accidental influences); some are anomalous (valid, but at odds with predictions or retrodictions); some are irreproducable and thus invalid; and some are irrelevant.
4. Sociological Criteria of Scientific Theories
A scientific theory must:
- resolve known problems, paradoxes, and/or anomalies that scientists haven't been able to deal with using past theories
- create new problems and questions to work on
- create a new paradigm or model to use when working on problems
- provide concepts which help scientists deal with problems
Some critics of science see the above criteria as problems, but they underscore how science is done by a community of researchers and that many scientific problems are discovered by the community. A scientific theory must address a genuine problem and must offer a means of resolving it. If there is no actual problem, how can a theory qualify as scientific?
5. Historical Criteria of Scientific Theories
A scientific theory must:
- meet or surpass the criteria of earlier theories or demonstrate that the criteria are artifactual and so should be replaced
- explain any and all data produced with earlier theories
- be consistent with any and all related theories
A scientific theory does not just solve a problem, but must do so in a way that is superior to other, competing theories. It must explain more data than the competition; scientists prefer fewer theories which explain more rather than many theories, each of which explains little. It should also not conflict with related theories that are clearly valid. This ensures that scientific theories increase in their explanatory power.
6. Legal Criteria of Scientific Theories
Root-Bernstein does not list legal criteria for scientific theories. Ideally there wouldn't be, but Christians have made science a legal issue. In 1981 an Arkansas trial over "equal treatment" for creationism in science classes was overturned and the U.S. Supreme Court ruled such laws were unconstitutional. In his ruling Judge Overton said science has four essential features:
- It is guided by natural laws, and is explanatory by references to natural laws
- Science is testable against the empirical world
- Its conclusions are tentative, not the final word
- It is falsifiable
In the U.S., then, there is a legal basis for answering the question, "what is science?"
7. Summary of Criteria of Scientific Theories
The criteria for scientific theories can be summarized by these principles:
- Consistent (internally & externally)
- Parsimonious (sparing in proposed entities, explanations)
- Useful (describes & explains observed phenomena)
- Empirically Testable & Falsifiable
- Based upon Controlled, Repeated Experiments
- Correctable & Dynamic (changes are made with new data)
- Progressive (achieves all that previous theories have and more)
- Tentative (admits that it might not be correct, does not assert certainty)
These criteria are what we expect for a theory to be considered scientific. Lacking one or two might not mean a theory isn't scientific, but only with good reasons. Lacking most or all is a disqualification.