Science requires that scientific explanations of phenomena be based on events or mechanisms that can be observed in the natural world. This is how science builds a base of shared observations and ideas to which new knowledge can be added. For example, scientists studying the characteristics of plants and animals in Hawaii look for natural explanations for those characteristics. They propose hypotheses that explain the evolution of those characteristics through naturally occurring mechanisms. Then they gather additional information to test their hypotheses. Because hypotheses are based on phenomena that can be measured or observed, other scientists can test the hypotheses by gathering their own data. Based on the evidence gathered, the hypothesis can be accepted or rejected and new, more refined hypotheses can be developed. One potential source of confusion in discussing the theory of evolution is the meaning of the words “theory,”“hypothesis,” and “fact.” In popular usage, a “theory” is something that is not known for sure. But the word “theory” has a very different meaning in science than it does in everyday use. In science, “theory” refers to an explanation of some aspect of the natural world that is held with great confidence because it is supported by overwhelming evidence. The theory of gravitation holds that all objects are attracted to each other in proportion to their mass. The cell theory says that all living things are composed of cells. Scientists use the word “hypothesis” to describe an idea or model that has not yet been fully tested. For example, in studying evolution in Hawaii, a scientist might hypothesize that a species on one island is descended from a species on another island. The scientist then would gather evidence to test that hypothesis. If a hypothesis is supported by the evidence, the hypothesis may contribute to more complex explanations, including theories. If the available evidence does not support a hypothesis, that hypothesis can be rejected, modified, or subjected to further testing. For a hypothesis to fall within the realm of science, it must be constructed in such a way that it potentially can be shown to be wrong—otherwise the hypothesis cannot be tested against evidence from the natural world. This demand that a hypothesis be “falsifiable” is one of the defining characteristics of scientific explanations. A “fact,” in scientific terms, is an observation that has been repeatedly confirmed by the studies of different independent scientists. In other words, it is a phenomenon that has been observed so frequently that its existence is no longer being questioned. Because theories explain facts, they embody a greater understanding of the natural world than do observations. Without theories to explain and integrate them, facts become collections of unrelated observations. Evolutionary theory is a comprehensive explanation that integrates facts from many different areas of science. It has proven tremendously successful in explaining the basis for observed phenomena and in allowing scientists to make predictions based on existing data. Fact: In science, an observation that has been repeatedly confirmed. Hypothesis: A testable statement about the natural world that can be used to build more complex inferences and explanations. Theory: In science, a well-substantiated explanation of some aspect of the natural world that can incorporate facts, laws, inferences, and tested hypotheses. (Adapted from Teaching About Evolution and the Nature of Science.)
Next, we consider the basic structure of the most comprehensive and effective deployment of inductive reasoning in human history. Since its development during the Renaissance, modern science has contributed significantly to our ability to perceive, understand, and manipulate the natural world. Taken generally as a way of acquiring human knowledge, science is a procedure for the invention and evaluation of hypotheses that may be used to explain why things happen as they do. Unlike dogmatic appeals to the absolute, unchallengeable truth of unsupported assertions (as, for example, when a parent tells a child, "Because I say so, that's why."), scientific explanations are always tentative proposals, offered in hopes of capturing the best outlook on the matter but subject to evaluation, modification, or even overturn in light of further evidence. The most productive model for the structure of a scientific explanation is that of a valid deductive argument whose conclusion is the event to be explained. Some of the premises of this argument will be factual statements of the antecedent circumstances, while the others will be the scientific hypotheses offered as a way of linking those circumstances to the outcome stated by the conclusion. Scientific predictions have exactly the same structure; the only difference between the explanation and the prediction of an event is whether or not it has already occurred. On this deductive-nomological model for scientific explanation, the conclusion of the argument must be true (that is, the event must occur) if all of the premises are true. Those of its premises that state the antecedent circumstances will naturally be true so long as we have our facts straight. But the truth of the hypotheses, which try to capture the lawlike relationship between those circumstances and the event to be explained, will always remain open to question. So the quality of the explanation as a whole typically rests upon the extent to which these hypotheses are reliable. This reliability can never be established with absolute certainty. It is sometimes possible to eliminate bad hypotheses by using them as the premises of a deductive argument predicting that particular consequences will follow from a particular set of circumstances and then showing that the predicted event does not, in fact, occur. (This amounts to the use of Modus Tollens to show that since the consequent is false, some part of the antecedent must also be false.) But if the events turn out as predicted, that only tends to confirm the hypotheses; it cannot prove their truth. (Since that would amount to reliance on the fallacy of affirming the consequent.) Empirical evidence typically underdetermines scientific explanation, leaving us with multiple hypotheses, any one of which would account for the facts. Evaluating HypothesesAlthough it always remains impossible in principle to prove the truth of a scientific hypothesis, it is possible to compare the distinct hypotheses involved in rival explanations of the same event. Here are several criteria that can bu used in making these relative judgments about the reliability of any hypothesis offered as part of a scientific explanation:
The Scientific MethodWhat is often called the scientific method is nothing more than a step-by-step procedure for the conduct of scientific research:
The Uses of ExperimentationNotice that although scientific method is properly considered to rely upon empirical knowledge, several steps in the process do not appeal to any direct observation of the facts. Steps 1, 2, and 4 in particular often involve creative leaps of intution that are not warranted by the evidence. Novel ways of defining a problem and radically different hypotheses about solving it typically arise from insight and imagination rather than from observation. But even in such cases, the scientists differs from dogmatists in their determination to put every such hypothesis to the test in steps 5 and 6. Usually experimentation confirms a hypothesis by eliminating its likely alternatives. Thus, the most powerful confirmation occurs when we are able to devise a "crucial experiment," a set of circumstances from which rival hypotheses predict distinct results: when we perform such a test, one hypothesis nearly always emerges as the most likely. But this situation tends to arise only within the context of a well-developed general theory with which only a few alternative hypotheses would be relevant. As we've noted before, scientific revolutions can occur only when we step outside the bounds of such a restrictive experimental framework. It is worth noticing that even apparently factual, "objective" investigations of the natural world often rely upon the theoretical background of a set of accepted scientific hypotheses. Thus, for example, biological taxonomy and descriptions of historical events usually employ formal systems of classification that embody significant hypotheses about presumed similarities of nature or origin. Here, as in all of our efforts to engage in sound reasoning, it is vital to recognize and uncover the implicit foundations of what we believe.
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