Many people, even those who have successfully completed science courses, have incorrect or misleading ideas about some of the most important and basic principles in science. In particular, the terms "fact" and "belief" are often used incorrectly, as are "theory," "hypothesis," and "law." The following paragraphs will define and distinguish these terms, so that they can be correctly interpreted in the remaining sections of this text.
Beliefs and Facts
A belief is a statement that is not scientifically-provable. In contrast, a fact is a basic statement that is established by experiment or observation. While all facts are true under the specific conditions of the observation, beliefs cannot be proved as either correct or incorrect.
Theories, Hypotheses, and Laws
The following is the description of a theory, as defined by the United States National Academy of Sciences.
"Some scientific explanations are so well established that no new evidence is likely to alter them. The explanation becomes a scientific theory. In everyday language a theory means a hunch or speculation. Not so in science. In science, the word theory refers to a comprehensive explanation of an important feature of nature supported by facts gathered over time. Theories also allow scientists to make predictions about as yet unobserved phenomena."
"A scientific theory is a well-substantiated explanation of some aspect of the natural world, based on a body of facts that have been repeatedly confirmed through observation and experimentation. Such fact-supported theories are not "guesses" but reliable accounts of the real world. The theory of biological evolution is more than "just a theory." It is as factual an explanation of the universe as the atomic theory of matter (stating that everything is made of atoms) or the germ theory of disease (which states that many diseases are caused by germs). Our understanding of gravity is still a work in progress. But the phenomenon of gravity, like evolution, is an accepted fact."
Based on this definition, a theory is an explanation of natural phenomenon and is not a prediction or an "educated guess," which are the parameters of a hypothesis. Because theories have so much support and are able to explain a wide variety of observations, theories are unlikely to change. Scientific laws are similar to scientific theories in that both are principles that can be used to predict the behavior of the natural world. Both scientific laws and scientific theories are typically well-supported by observations and/or experimental evidence. While theories are overarching explanations of how nature works and why it exhibits certain characteristics, laws refer to the rules that dictate how nature will behave under certain conditions.