Hypothesis, theory, fact, law. Prefaced with “hunch” or “guess”, this list of terms would reflect what many people consider a graded series from least to greatest degree of certainty. This ranking may be appropriate in common usage, but actually makes little sense when these words are employed in a scientific context.
Fact
“Fact” is perhaps the only term in the above list whose common and technical definitions are similar. The major difference is in the degree of certainty expressed, which is simultaneously higher and lower in scientific usage. Following the definition provided by the US National Academy of Science (NAS) (1998), one of the most prestigious scientific societies in the world, a scientific fact is “an observation that has been repeatedly confirmed, and for all practical purposes, is accepted as ‘true’.” Or, as Stephen Jay Gould (1981) put it in his inimitable style, “In science, ‘fact’ can only mean ‘confirmed to such a degree that it would be perverse to withhold provisional assent’.” It is this insistence on repeated confirmation by data—either through direct observation or reliable inference—that makes a claim to “fact” so much stronger in science. However, as the NAS points out, “truth in science is never final, and what is accepted as a fact today may be modified or even discarded tomorrow”. Small-scale details are regularly revised as more precise observations are made, whereas well established facts of fundamental significance are very rarely overthrown, but in principle, no scientific fact of any magnitude is beyond revision or refutation. As a result, scientists must maintain a balance between the confidence that comes from reinforcing conclusions about the world with repeatable data and the understanding that absolute certainty is not something that the methods of science are able or intended to deliver.
Theory
The common and scientific definitions of “theory,” unlike of “fact,” are drastically different. In daily conversation, “theory” often implicitly indicates a lack of supporting data. Indeed, introducing a statement with “My theory is...” is usually akin to saying “I guess that...”, “I would speculate that...”, or “I believe but have not attempted to demonstrate that...”. By contrast, a theory in science, again following the definition given by the NAS, is “a well-substantiated explanation of some aspect of the natural world that can incorporate facts, laws, inferences, and tested hypotheses.” Science not only generates facts but seeks to explain them, and the interlocking and well-supported explanations for those facts are known as theories. Theories allow aspects of the natural world not only to be described, but to be understood. Far from being unsubstantiated speculations, theories are the ultimate goal of science.
Hypothesis
The validity of scientific theories is not determined solely by their ability to accommodate and account for known facts. Theories also are actively tested, and it is here that “hypotheses” play an important role. According to the NAS, a scientific hypothesis is “a tentative statement about the natural world leading to deductions that can be tested.” Testing can involve direct experimentation or the generation of predictions about as-yet-unobserved facts that can be evaluated by further observation. This latter process plays a significant part in the validation of theories in sciences such as astronomy and geology where direct experimental manipulation is difficult. As the NAS notes, “If the deductions are verified, the hypothesis is provisionally corroborated. If the deductions are incorrect, the original hypothesis is proved false and must be abandoned or modified.” It bears noting that the rejection of a hypothesis does not automatically imply the refutation of an entire theory because hypotheses are usually sufficiently focused to test only one aspect of complex theories.
Law
Finally, “law”, for which, once again, there is a nearly opposite definition in everyday use compared to the application of the term in science. “Laws,” in normal experience, are prescriptive—that is, they dictate what behaviors one should carry out and which ones must be avoided. A posted speed limit, for example, (attempts to) dictate the behaviors of drivers. A scientific law, on the other hand, is descriptive—it is a “generalization about how some aspect of the natural world behaves under stated circumstances” according to the NAS. In the vernacular, a law prescribes behavior and limits what is permitted to happen. In science, a law describes and predicts what will happen when the range of possible conditions is limited. If one is caught speeding, then mechanisms are implemented to correct this deviation from externally imposed limits. However, there is no punishment for “violating the laws of physics” or “defying the law of gravity” because these phrases are nonsensical from a scientific standpoint.
More specifically, if an observation does not conform to the expectations of a scientific law, then either (1) the observation was illusory or interpreted incorrectly, (2) the observed event took place outside the specified conditions to which the law applies, or (far less likely), (3) the law is inaccurately formulated. A prime example is provided by the chronically misunderstood Second Law of Thermodynamics, which states that “the entropy of a closed system not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium.” In this case, the conditions are very clearly specified: if there is no external source of energy (“a closed system”), then there will be a net increase in disorder until the system reaches equilibrium. Local increases in order are not precluded (ornate snowflakes still form from water vapor), and of course, this does not apply to living things, which draw energy from their environments (and ultimately from the sun), and hence, represent open systems. Readers of this article establish this latter claim conclusively, having passed from a simple zygote to a complex organism composed of trillions of specialized cells. If the Second Law of Thermodynamics implied that all natural increases in order were impossible, then it would be incorrect. It does not and (so far as we know) is not. The broader point is that invoking the Second Law of Thermodynamics as an argument against evolution reveals a misunderstanding of both the scope of this particular law and of the meaning of “law” in science generally.
Theories explain facts and are tested by generating hypotheses. No matter how much information accrues, hypotheses never become theories, and theories never graduate into laws. These terms describe three distinct aspects of science.