In contrast to theories that are intended to explain phenomena, laws serve to describe them. Scientific laws describe observed universal regularities in nature, For example, the laws of thermodynamics describe what happens when heat is transformed in mechanical energy, while theories of thermodynamics explain this event. The elevation of a principle to law in a discipline usually takes a long time to achieve, and during which the principle is tested and verified. However, there tends to be confusion about the differences in meaning between a law and a principle. A description of nature that covers more than one law, but has not achieved the status of a law, is sometimes called a theory. Especially in social sciences, laws and principles are used in a much less rigorous fashion and may simply imply that something occurs regularly under certain conditions. Psychology, despite announcements in the past that one or other law has been found, has been particularly resistant to them. Finally, a law is not the same as a rule.
See Behaviorism, Bridge law (or principle), Classical thermodynamics, Description, Dynamical systems approaches, Emergence, Explanation, Isomorphism, Law-like statement, Laws of nature, Ontogenetic development, Paradigm, Principle, Rule, Theory