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The words hypothesis, law, and theory refer to different kinds of statements, or sets of statements, that scientists make about natural phenomena. A hypothesis is a proposition that attempts to explain a set of facts in a unified way. It generally forms the basis of experiments designed to establish its plausibility. Simplicity, elegance, and consistency with previously established hypotheses or laws are also major factors in determining the acceptance of a hypothesis. Though a hypothesis can never be proven true (in fact, hypotheses generally leave some facts unexplained), it can sometimes be verified beyond reasonable doubt in the context of a particular theoretical approach. A scientific law is a hypothesis that is assumed to be universally true. A law has good predictive power, allowing a scientist (or engineer) to model a physical system and predict what will happen under various conditions. New hypotheses inconsistent with well-established laws are generally rejected, barring major changes to the approach. An example is the law of conservation of energy, which was firmly established but had to be qualified with the revolutionary advent of quantum mechanics and the uncertainty principle. A theory is a set of statements, including laws and hypotheses, that explains a group of observations or phenomena in terms of those laws and hypotheses. A theory thus accounts for a wider variety of events than a law does. Broad acceptance of a theory comes when it has been tested repeatedly on new data and been used to make accurate predictions. Although a theory generally contains hypotheses that are still open to revision, sometimes it is hard to know where the hypothesis ends and the law or theory begins. Albert Einstein's theory of relativity, for example, consists of statements that were originally considered to be hypotheses (and daring at that). But all the hypotheses of relativity have now achieved the authority of scientific laws, and Einstein's theory has supplanted Newton's laws of motion. In some cases, such as the germ theory of infectious disease, a theory becomes so completely accepted, it stops being referred to as a theory.