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In science and philosophy, an ad hoc hypothesis is a hypothesis added to a theory in order to save it from being falsified. Ad hoc hypothesizing is compensating for anomalies not anticipated by the theory in its unmodified form.
In the scientific community
Scientists are often skeptical of theories that rely on frequent, unsupported adjustments to sustain them. This is because, if a theorist so chooses, there is no limit to the number of ad hoc hypotheses that they could add. Thus the theory becomes more and more complex, but is never falsified. This is often at a cost to the theory's predictive power, however. Ad hoc hypotheses are often characteristic of pseudoscientific subjects.
An ad hoc hypothesis is not necessarily incorrect; in some cases, a minor change to a theory was all that was necessary. For example, Albert Einstein's addition of the cosmological constant to general relativity in order to allow a static universe was ad hoc. Although he later referred to it as his "greatest blunder", it may correspond to theories of dark energy.
Naturally, some gaps in knowledge, and even falsifying observations must be temporarily tolerated while research continues. To temper ad hoc hypothesizing in science, common practice includes falsificationism (somewhat in the philosophy of Occam's razor). Falsificationism means scientists become more likely to reject a theory as it becomes increasingly burdened by ignored falsifying observations and ad hoc hypotheses.