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with well-accepted hypotheses in the relevant discipline are likely to be ignored when they are not availed by convincing empirical evidence. The microhistory of science is littered with farfetched or ad hoc hypotheses, often proposed by individuals with no previous or posterior scientific achievements. Theories of this sort usually fade away because they are ignored by most of the scientific community, although on occasion they engage their interest because the theory may have received attention from the media or even from political or religious bodies. The fiasco 2 decades ago over “cold fusion” was an example of an unlikely and poorly tested hypothesis that received some attention from the scientific community because its proponents were well-established scientists (Taubes, 1993).

The fourth and most distinctive step in testing a scientific hypothesis consists of putting the hypothesis on trial by ascertaining whether or not predictions about the world of experience derived as logical consequences from the hypothesis agree with what is actually observed. This is the critical element that distinguishes the empirical sciences from other forms of knowledge: the requirement that scientific hypotheses be empirically falsifiable. Scientific hypotheses cannot be consistent with all possible states of affairs in the empirical world. A hypothesis is scientific only if it is consistent with some but not with other possible states of affairs not yet observed in the world, so that it may be subject to the possibility of falsification by observation. The predictions derived from a scientific hypothesis must be sufficiently precise that they limit the range of possible observations with which they are compatible. If the results of an empirical test agree with the predictions derived from a hypothesis, the hypothesis is said to be provisionally corroborated; otherwise it is falsified.

The requirement that a scientific hypothesis be falsifiable has been appropriately called the criterion of demarcation of the empirical sciences because it sets apart the empirical sciences from other forms of knowledge (Popper, 1959, 1963). A hypothesis that is not subject to the possibility of empirical falsification does not belong in the realm of science.

The requirement that scientific hypotheses be falsifiable rather than simply verifiable seems surprising at first. It might seem that the goal of science is to establish the “truth” of hypotheses rather than attempt to falsify them, but it is not so. There is an asymmetry between the falsifiability and the verifiability of universal statements that derives from the logical nature of such statements. A universal statement can be shown to be false if it is found to be inconsistent with even 1 singular statement, i.e., a statement about a particular event. But, a universal statement can never be proven true by virtue of the truth of particular statements, no matter how numerous these may be.

Consider a particular hypothesis from which a certain consequence is logically derived. Consider now the following argument: If the hypothesis



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