The Hypothetico Deductive Method

Scientific theories can be tested by the hypothetico-deductive method. First, a hypothesis is postulated: that the Earth orbits around the Sun. Then, a prediction is deduced from the hypothesis: the motion of the Earth will be revealed in a stellar parallax. Then the test: is a stellar parallax found?

In this historical case, the answer was no; and the logic of the method dictated that the original hypothesis was thus refuted: the Copernican theory was false. Or an auxiliary hypothesis had been overlooked: that the stars are very distant and thus a parallax angle may actually exist but is too small to detect.

Recourse to an auxiliary hypothesis in an attempt to rescue a cherished major hypothesis after it has been observationally refuted constitutes an ex post facto argument. In science, such arguments made after a fact (in this case, no stellar parallax) becomes known or a theory is modified to bring it into agreement with new facts, are less psychologically compelling than are predictions of previously unknown phenomena, even if the strict logic of the two situations is equally compelling.

The hypothetico-deductive method has the potential—logically, at least in principle—to refute a hypothesis, if all potentially extenuating auxiliary assumptions are included in the analysis.

But the hypothetico-deductive method cannot be used to prove a hypothesis. When a theory is tested using several predictions, and all the predicted phenomena are found, one reasonably gains confidence in the theory. But observations do not (can never) constitute absolute proof. When observations match prediction, a theory commands greater confidence, but a theory is never (can never be) incontrovertibly proven.

In attempting to prove the Copernican theory, consider the following logic:

hypothesis: the Copernican theory prediction: if this theory, then Mercury and Venus will always be near the Sun observation: Mercury and Venus are always observed near the Sun conclusion: therefore the Copernican theory

But is this conclusion warranted? Aren't Mercury and Venus always seen near the Sun in the Ptolemaic model too? We have here a case of the fallacy of affirming the consequent: If A, then B; B; therefore A.

Remember the medieval nominalist argument: that we cannot insist upon the truth of any particular working hypothesis because God could have made the world in some different manner that nonetheless has the same set of observational consequences.

A God capable of creating the world in a different manner isn't even necessary: just some ingenious person to imagine such a possibility. Ptolemy kept Mercury and Venus always near the Sun, but not as an inevitable consequence of his geometric model. The desired naturalness and inevitability can, however, be achieved in an Earth-centered system. Were the Sun to orbit around a motionless Earth in the center while Mercury and Venus orbited around the Sun, Mercury and Venus would always be seen near the Sun. Indeed, just such a model would be proposed as a rival to the Copernican theory.

An unauthorized preface added to Copernicus's De revolutionibus presented Copernicus's theory as a mathematical fiction rather than the true account of the beauty and harmony of the universe that Copernicus intended. The spurious preface read, in part:

For it is the job of the astronomer to use painstaking and skilled observation in gathering together the history of the celestial movements, and then—since he cannot by any line of reasoning reach the true causes of these movements—to think up or construct whatever causes or hypotheses he pleases such that, by the assumption of these causes, those same movements can be calculated from the principles of geometry for the past and for the future too. . . . [I]t is not necessary that these hypotheses should be true, or even probable; but it is enough if they provide a calculus which fits the observations . . . [L]et no one expect anything, in the way of certainty from astronomy, since astronomy can offer us nothing certain, lest if anyone take as true that which has been constructed for another use, he go away from this discipline a bigger fool than when he came to it. Farewell. (De revolutionibus, introduction)

Andrew Osiander, a Lutheran clergyman who tended Copernicus's book through the printing process, added this preface without Copernicus's knowledge. Copernicus saw the book only as he was dying, and was unable to make any changes.

Copernicus was a mathematical realist, not an instrumentalist or a nominalist. He believed passionately that the admirable symmetry and harmony in motions and magnitudes contained within his astronomical model was proof of its reality. Observed phenomena that were ad hoc arbitrary arrangements in Ptolemy's geocentric model occurred automatically and inevitably in Copernicus's heliocentric model. This element of completeness gave Copernicus confidence that he had discovered the real order of the cosmos, not merely a mathematical fiction.

Confidence and passion energized Copernicus and led to the scientific breakthrough only hinted at in the earlier hypothetical musings of nominalists such as Buridan and Oresme. Copernicus the realist exalted:

In the middle of all sits Sun enthroned, in this most beautiful temple could we place this luminary in any better position from which he can illuminate the whole at once? He is rightly called the Lamp, the Mind, the Ruler of the Universe . . . So the Sun sits upon a royal throne ruling his children the planets which circle round him. The Earth has the Moon at her service. . . . [T]he Earth conceives by the Sun, and becomes pregnant with an annual rebirth. (De revolutionibus, I10)

For all that Copernicus accomplished, his astronomy remained the astronomy of the Greeks and of Ptolemy, employing combinations of uniform circular motions to save the phenomena. A simple change in geometry, switching the Earth and the Sun, had little affect on working astronomers. Either system

Copernicus's Grave

Copernicus was buried in a tomb under the floor of the Roman Catholic cathedral in Frombork, 180 miles north of Warsaw. In the summer of 2005, after a year of searching, Polish archeol-ogists located what is very likely Copernicus's grave. Police forensic experts determined that the skull belonged to a man who died at about age 70, and a computer reconstruction of the face matched portraits of Copernicus with a broken nose and a scar above his left eye. Scientists will try to find relatives of Copernicus and do DNA identification.

r----------------------------------i i Mental Exercise: Greater and Lesser Progressions and i

1 Retrogressions 1

I In addition to distances at conjunction and opposition, there also followed I

I naturally and inevitably from Copernicus's theory explanations for "why I

' the progression and retrogression appear greater for Jupiter than Saturn, '

and less than for Mars, but again greater for Venus than for Mercury; and

I why such oscillation appears more frequently in Saturn than in Jupiter, but .

I less frequently in Mars and Venus than in Mercury. . . ." (De revolutioni- |

I bus, I10). Explain, with the use of diagrams, how these phenomena follow | inevitably from Copernicus's heliocentric model.

could be used for calculating planetary positions. Neither system then commanded an observational advantage over the other. De revolutionibus was not a revolutionary book. But it was revolution-making. How switching the positions of the Sun and Earth initiated a scientific revolution is the subject of the following chapter.

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