Evolution has been described as "fact and theory", "fact not theory", "only a theory, not a fact", "multiple theories, not fact", and neither "fact, nor theory". The disagreements among these statements, however, has little bearing on the relative veracity of evolution. Different researchers have different reasons in the nuances of their philosophical outlook that leads to different claims. For example, Fitzhugh (2007) claimed that evolution is neither fact nor theory, because such a statement is too general to carry proper meaning or understanding of the specific things or processes being referenced. "To say ‘evolution is a fact’ is just an inexact reference to what is thought to have existed, which are organisms and the events in which they were involved." In contrast, however, Gould (1981) claimed that Charles Darwin developed a testable theory of mechanism (e.g., natural selection) that caused the manifest fact of evolution to occur. The reason for these different claims in reference to evolution as fact and theory is that Gould (1981) refers to evolution in broader terms, whereas Fitzhugh (2007) emphasizes the importance of constructing theory in the strictest sense for testing explicit hypotheses through specific reference to the facts in need of explanation.
Science cannot achieve absolute "certainty" nor is it a continuous march toward an objective truth as the vernacular meaning of the terms "proof" or "fact" might imply. A proof, fact, theory, hypothesis, and other words of science are hobbled by multiple meanings but are used nonetheless because they invigorate research methods and lead to discovery in all branches of scientific research. The philosophy of scientific inquiry solves problems of novelty as discoveries are made. Scientific knowledge is shared, incorporated, and tested across disciplines. Charles Darwin, for example, not only advanced theory and hypotheses in evolution, but experimented and tested his ideas across disciplines, including and not limited to geology, botany, psychology, and ecology.
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