As with any scientific theory, the modern synthesis is constantly debated, tested, and refined by scientists, but there is an overwhelming consensus in the scientific community that it remains the only robust model that accounts for the known facts concerning evolution. Critics also state that evolution is not a fact. In science a fact is a verified empirical observation while in colloquial contexts a fact can simply refer to anything for which there is overwhelming evidence. For example, in common usage theories such as "the Earth revolves around the Sun" and "objects fall due to gravity" may be referred to as "facts", even though they are purely theoretical. From a scientific standpoint, therefore, evolution may be called a "fact" for the same reason that gravity can: under the scientific definition, evolution is an observable process that occurs whenever a population of organisms genetically changes over time. Under the colloquial definition, the theory of evolution can also be called a fact, referring to this theory's well-established nature. Thus, evolution is widely considered both a theory and a fact by scientists. Similar confusion is involved in objections that evolution is "unproven", since no theory in science is known to be absolutely true, only verified by empirical evidence. This distinction is an important one in philosophy of science, as it relates to the lack of absolute certainty in all empirical claims, not just evolution. Strict proof is possible only in formal sciences such as logic and mathematics, not natural sciences (where terms such as "validated" or "corroborated" are more appropriate). Thus, to say that evolution is not proven is trivially true, but no more an indictment of evolution than calling it a "theory". The confusion arises in that the colloquial meaning of proof is simply "compelling evidence", in which case scientists would indeed consider evolution "proven". Degree of acceptance An objection is often made in the teaching of evolution that evolution is controversial or contentious. Unlike past creationist arguments which sought to abolish the teaching of evolution altogether, this argument makes the claim that evolution should be presented alongside alternative views since it is controversial, and students should be allowed to evaluate and choose between the options on their own. This objection forms the basis of the "Teach the Controversy" campaign by the Discovery Institute, a think tank based in Seattle, Washington, to promote the teaching of intelligent design in U.S. public schools.