Returning to Ibn al-Haitham's critique of Ptolemy's Almagest, I will quote his conclusion at some length, not only because it draws the real demarcation lines of the new astronomy Ibn al-Haitham was calling for, but also because it demonstrates the utter dissatisfaction that was obviously felt with Greek astronomy. No one could possibly chart the contours of the new astronomy, or express the sentiments of dissatisfaction with the old, better than Ibn al-Haitham himself. In his own words:
We must elucidate the method that was followed by Ptolemy for determining the configurations of the planets. That is, he had gathered together all the motions of the individual planets that he could verify with his own observations, or the observations of those who had preceded him. He then sought a configuration that could possibly exist for real bodies that moved with those motions, and was not able to achieve it. He then assumed an imaginary configuration with imaginary lines and circles that could move in those motions, even though only some of those motions could indeed take place in [real] bodies that moved in those motions. He was obliged to follow that route for he could not devise another.
But if one were to assume an imaginary line, and made that line move in his imagination, it would not follow that there should be a corresponding line that would move in the heavens with that motion. Nor would it be true that if one imagined a circle in the heavens, and imagined a planet to move on that circle, that the [real] planet would [in fact] move along that imaginary circle. And if that were so, then the configurations that were assumed by Ptolemy for the five planets were false configurations, and that he had established them after he knew that they were false, for he was unable to obtain others. The motions of the planets, however, have correct configurations in [real] existent bodies that Ptolemy did not come to understand nor could he achieve. For it is not admissible that a perceptible, perpetual and uniform motion be found without it having a correct configuration in [real] existent bodies. This is all we have regarding the book of the Almagest.[201]
With this summary condemnation of Ptolemaic astronomy, Ibn al-Haitham was obviously setting the field of Arabic astronomy on completely new footing. He could not stress any more forcefully the need for the consistency between the assumptions about the nature of the bodies that constitute the universe, and the construction of mathematical models for planetary motions that could represent those bodies without violating the very physical reality of the spheres of which the world was supposed to be made. That is the most succinct statement of the principle of consistency that was to characterize the new astronomy from that time on.
Put briefly, it should be clear that one does not accept a set of principles regarding the physical formation of the universe, and then develop mathematical models to illustrate the behavior of that universe in such terms that would contradict the very physicality of the objects that were originally accepted, or transform them so that they would no longer be recognizable. It is like assuming the world is made of a sphere and then for purposes of demonstrating how it moves one ends up representing the world with the mathematical figure of a triangle.
Similar criticisms were also directed at the Ptolemaic texts in the earlier centuries, as was documented before, and some of them had hinted to this new approach of consistency between the physical world and its presumed behavior. But at no time before Ibn al-Haitham was this new understanding of the fundamentals of new astronomy so well articulated.
The text of the Planetary Hypotheses did not fare much better in Ibn al-Haitham's estimation, and most certainly did not advance the new ways of thinking about astronomy. In contrast to the Almagest, where one could find excuses for Ptolemy and claim that he was talking about imaginary circles and lines, i.e. abstract mathematical models, and not about real physical bodies whose motions would entail the absurdities enumerated, in the case of the Planetary Hypotheses Ptolemy spoke of physical bodies explicitly. Thus the type of criticism advanced by Ibn al-Haitham became much more pertinent with respect to that book. In addition, since the Planetary Hypotheses was written after the Almagest, Ibn al-Haitham then took advantage of that chronology, and seized the opportunity to compare Ptolemy's thinking about the subject at two different stages of his scientific career and in two different works. He combed the second work, the Planetary Hypotheses, in order to determine if the absurdities of the Almagest had by then been resolved.
Surprisingly, he found out that the problems became much worse. Instead of resolving some of the outstanding problems of the Almagest, Ptolemy added some new ones in the Planetary Hypotheses.
Ibn al-Haitham went through both texts and produced a comparative list of spheres and motions that were described in the Almagest, and were now changed in the Planetary Hypotheses. While the configuration that was drawn for the sun remained the same in the two texts, and while the motions of the moon were nominally also the same, the motion that was described in the Almagest as producing the correction for the prosneusis phenomenon was not mentioned in the Planetary Hypotheses. In the case of Mercury only five of its motions that were mentioned in the Almagest were retained, and three were dropped. Similarly in the case of Venus, four motions were retained and three were dropped. The upper planets retained all the motions that were described in the Almagest and only the latitude motion around the small circles was dropped. But there were some more drastic changes made in the rest of the arrangement that Ptolemy had stipulated for the motion of the planets in latitude.
After going through this comparative survey in some detail, Ibn al-Haitham reached the preliminary conclusion that the configurations that were described in the Planetary Hypotheses were different from those described in the Almagest, if for no other reason except that some ten motions were no longer mentioned in the new text and the motion in latitude was overhauled. To which Ibn al-Haitham says:
This arrangement, which was detailed in the first treatise of the Planetary Hypotheses is contrary to the one that was proposed in the Almagest, and it is also contrary to the observed latitudinal motions of the planets to the north or to the south when they were close to their epicyclic apogee. Then it becomes evident that the configuration that is described in the first treatise of the Planetary Hypotheses is not only contrary to observation, but that it was also contrary to what he had established in the Almagest.[202]
After a thorough study of the various motions that were described in the Planetary Hypotheses, and their causes, Ibn al-Haitham found himself quoting Ptolemy, in several passages, where Ptolemy would be caught saying that all those motions should be accounted for by real spherical bodies that were responsible for them. That left Ibn al-Haitham with one conclusion: that Ptolemy had explicitly committed himself to "finding for every motion that was mentioned in the Almagest a corresponding body that moved by that motion."[203]
As for obvious contradictions, even in the same book, those were definitely used as further fodder to support Ibn al-Haitham's thesis. To give just one example of the kind of issues Ibn al-Haitham emphasized, he noted that in the second treatise of the Planetary Hypotheses Ptolemy had said that motion by compulsion was not permissible in the celestial spheres, when he had already said in the first treatise that each one of those spheres would have a motion of its own and another one that was forced upon it.[204]