15.6.2: Retaining Hypotheses Despite Negative Test Results
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If a scientist puts a hypothesis to the test, and if the test produces results inconsistent with the hypothesis, there is always some way or other for the researcher to hold onto the hypothesis and change something else. For example, if the meter shows "7" when your hypothesis would have predicted "5," you might rescue your hypothesis by saying that your meter wasn't working properly. However, unless you have some good evidence of meter trouble, this move to rescue your hypothesis in the face of disconfirming evidence commits the fallacy of ad hoc rescue. If you are going to hold onto your hypothesis no matter what, you are in the business of propaganda and dogma, not science. Psychologically, it is understandable that you would try to rescue your cherished belief from trouble. When you are faced with conflicting data, you are likely to mention how the conflict will disappear if some new assumption is taken into account. However, if you have no good reason to accept this saving assumption other than that it works to save your cherished belief, your rescue is an ad hoc rescue.
Say why this is not a successful argument:
People such as Galileo will tell you that the Earth turns, but it really doesn't. If the Earth did turn, then when you drop a ball off the top of a tall building, the Earth would turn away from the ball, and the ball would land some distance away from the building. Instead, when you try this, the ball lands right at the base of the building. Therefore, the Earth doesn't really turn after all.
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In any good scientific test, the predicted outcome of the test will follow from the hypothesis being tested. It doesn't here. A faulty prediction was made from the hypothesis that the Earth turns. The Earth does turn; however, the ball and the building it is dropped from are both with the same angular velocity, so the dropped ball should merely go straight down, as in fact it does.
Back in the times of ancient Greece and Rome, augurs would advise the rulers about the future. These respected priest-prophets carried a staff or wand and specialized in foretelling events by using omens, or unusual events. Because the universe is made for people, anything unusual must be a sign, a special message that people are supposed to interpret, or so the augurs believed. They would try to predict the future for their rulers by interpreting the unusual flight of a bird, the shape and markings of the guts of sacrificed animals, and the appearance of comets and eclipses. Often, when their divining with ravens, livers, and comets was obviously not working and the ruler was complaining, the augurs would blame their failure on the negative influence from nearby Christians. Their solution was to ask the ruler to order the deaths of all the Christians. Examining this story from the perspective of scientific reasoning, we see that the principal mistake of the augurs was
a. that they should have relied more on scientific astrology.
b. their insensitivity to pseudoprecision.
c. to use the method of ad hoc rescue.
d. to overemphasize repeatable phenomena.
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Answer (c). Their hypothesis was that examining omens would enable them to foretell the future. Their predictions based on this hypothesis were inconsistent with the facts. To rescue their hypothesis, they revised it to say that omens could be used to predict the future provided Christians didn't interfere. However, there is no basis for believing that this revision is proper; its only basis is that if it were true, then the augurs could stay in business. So their reasoning commits the fallacy of ad hoc rescue.
In 1790 the French scientist Lavoisier devised a careful experiment in which he weighed mercury before and after it was heated in the presence of air. The remaining mercury, plus the red residue that was formed, weighed more than the original. Lavoisier had shown that heating a chemical in air can result in an increase in weight of the chemical. Today, this process is called oxidation. But back in Lavoisier's day, the accepted theory on these matters was that a posited substance, "phlogiston,” was driven off during any heating of a chemical. If something is driven off, then you would expect the resulting substance to weigh less. Yet Lavoisier's experiments clearly showed a case in which the resulting substance weighed more. To get around this inconsistency, the chemists who supported the established phlogiston theory suggested their theory be revised by assigning phlogiston negative weight. The negative-weight hypothesis was a creative suggestion that might have rescued the phlogiston theory. It wasn't as strange then as it may seem today because the notion of mass was not well understood. Although Isaac Newton had believed that all mass is positive, the negative-weight suggestion faced a more important obstacle. There was no way to verify it independently of the phlogiston theory. So, the suggestion appeared to commit the fallacy of ad hoc rescue.
An ad hoc hypothesis can be rescued from the charge of committing the fallacy of ad hoc rescue if it can meet two conditions: (1) The hypothesis must be shown to be fruitful in successfully explaining phenomena that previously did not have an adequate explanation. (2) The hypothesis's inconsistency with previously accepted beliefs must be resolved without reducing the explanatory power of science. Because the advocates of the negative-weight hypothesis were unable to do either, it is appropriate to charge them with committing the fallacy. As a result of Lavoisier's success, and the failure of the negative-weight hypothesis, today's chemists do not believe that phlogiston exists. And Lavoisier’s picture gets a prominent place in history: