# Word of the Day: Theory and Law

Let’s start with a few definitions within mathematics and logic.  An axiom or postulate is a proposition (statement) taken as a given.  A lemma is an intermediate proposition or stepping stone rather than the final result, which is a theorem.  A corollary follows readily from a theorem—it’s often simply another way of stating the theorem.  Lemmas, theorems, and corollaries are all proven, but proofs are only possible within mathematics and logic, not within science.

By contrast, all scientific statements are provisional.  A scientific hypothesis is a testable explanation for a phenomenon.  It explains and predicts.  Once a hypothesis has proven itself, it becomes a scientific theory.  A scientific law is a description of a natural phenomenon, often an equation.  Laws and theories are both well-tested, widely or universally accepted within the field, and falsifiable.  The main difference is that a theory explains while a law describes.

For example, germ theory, quantum theory, and the theory of evolution are explanations.  Boyle’s law, Ohm’s law, and Newton’s law of gravity are all descriptions (and are all equations).

A common misconception is that scientific hypotheses mature to theories, which mature to facts or laws.  Instead, facts (the observations from an experiment, for example) lead to hypotheses (a plausible but immature explanation), which lead to theories (well-evidenced explanations).  In the category of scientific explanations, a theory is as good as it gets and it doesn’t graduate to become a law.

Photo credit: Marvin (PA)

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## 3 thoughts on “Word of the Day: Theory and Law”

1. Good rundown. I think we need to keep harping on these ideas in order to take the wind out of the “it is only a theory” sails.

Corollary: As part of giving up belief in the supernatural, I have been challenging myself to change a lot of my vocabulary. Saying “hypothesis” instead of “theory” has been one such challenge.

2. Furthermore, I have taken to referring to “the natural-selection theory ABOUT evolution” to emphasize the distinction that evolution is an observed fact, and natural selection is the best explanation we’ve come up with for it.