As for the new physical bodies that were introduced by Ptolemy in the Planetary Hypotheses, namely the slices of spheres (manshūrāt) instead of the full spheres that were assumed in the Almagest, Ibn al-Haitham thought that the manshūrāt were a step in the wrong direction. For those slices in turn entailed "absurd impossibilities (muḥālāt fāḥisha), which are of two kinds: One takes place when the body empties one space to fill another, and the second when the body had to move in different and contrary motions."[205]
In the case of the full spheres that were assumed in the Almagest, they at least entailed "only one kind of impossibilities, and that is the different and contrary motions, and did not entail the other, namely, the emptying of one space and filling the other."[206] The example of the spheres, which had to move in different and contrary motions, is mentioned once more in connection with the equant problem that was already faced in the Almagest.[207]
Ibn al-Haitham's attitude toward those spherical slices of the Planetary Hypotheses were echoed two centuries later, in the work of Mu'ayyad al-Dīn al-'Urḍī (d. 1266), who also said that, as far as those spherical slices were concerned,
the impossibility that they would entail is even uglier (aqbaḥ) than that of the full spheres and more uncomely. For they would produce the same impossibilities mentioned before, like their moving non-uniformly around their own centers, and in addition they would entail orbs that were not spherical, but rather disconnected dissimilar surfaces, which is an impossibility in the natural sciences.[208]
Ibn al-Haitham had this to say about the motion in latitude, which Ptolemy had described in the Almagest by using a device of two small circles that would move the epicyclic radii, a feature that was dropped in the Planetary Hypotheses:
Then it becomes clear that Ptolemy was either in error when he disregarded the description of this configuration, or that he was wrong to establish this motion for the planets when he determined the latitudinal motion in the Almagest.[209]
Similarly, in the case of the inferior planets, Mercury and Venus, the small circles that were described in the Almagest to account for the motion of their epicycles in latitude, and which were now dropped in the Planetary Hypotheses had to lead to the conclusion that Ptolemy was either wrong in dropping them now, or in mentioning them in the Almagest in the first place. In whichever case, the treatment in the two books was contradictory, and that was one more obvious sign that the two books were read together.
Toward the end of the second treatise of the Planetary Hypotheses, Ptolemy seemed to lean toward the belief that it was possible to think of planets that would move by themselves, i.e. not to require a sphere that would move them. Ibn al-Haitham documented such statements very carefully only to conclude that not even the motion of rolling (tadaḥruj) should be permitted. For then
If Ptolemy could find it permissible that a planet could move by itself, without any body moving it, then that permissibility would make all the spherical slices as well as the spheres [themselves] invalid.[210]
In essence, Ibn al-Haitham was saying if planets could exhibit all those motions on their own, without any bodies moving them, then all of those assumptions of spheres and slices of spheres and the like would be completely superfluous. And here again 'Urḍī adopted a similar attitude in his own critique of Ptolemy in a slightly different context:
If one were to accept such impossibilities in this discipline (ṣinā'a), it would have been all in vain, and one would have found it sufficient to take only one concentric sphere for each planet, thus rendering eccentric and epicyclic spheres superfluous.[211]
Ibn al-Haitham concluded his critique of Ptolemy's Planetary Hypotheses with the following statement:
He [meaning Ptolemy] either knew of the impossibilities that would result from the conditions that he assumed and established, or he did not know. If he had accepted them without knowing of the resulting impossibilities, then he would be incompetent in his craft, misled in his attempt to imagine it and to devise configurations for it. And he would never be accused of that. But if he had established what he established while he knew the necessary results—which may be the case befitting him — with the reason being that he was obliged to do so for he could not devise a better solution, and [yet] he went ahead and knowingly delved into these contradictions, then he would have erred twice: once by establishing these notions that produce these impossibilities, and the second time by committing an error when he knew that it was an error.
To be fair, had all this been considered, Ptolemy would have established a configuration for the planets that would have been free from all these impossibilities, and he would not have resorted to what he had established — with all the resulting grave impossibilities — nor would he have accepted that had he been able to produce something better.
The truth that leaves no room for doubt is that there are correct configurations for the movements of the planets, which exist, are consistent, and entail none of these impossibilities and contradictions. But they are different from the ones that were established by Ptolemy. And Ptolemy could not comprehend them. Nor could his imagination attain their true nature.[212]
By moving from the critique of the Planetary Hypotheses directly to the Optics of Ptolemy, Ibn al-Haitham did not only demonstrate that the Greek astronomical tradition was essentially flawed but that the other sciences, like optics, suffered from the same inconsistencies as well. This is a clear indication of the pervasiveness of this critical spirit in Islamic times, and confirms what was stated before about the social motivations for such critiques that were by no means restricted to astronomy alone. Furthermore, it also demonstrates the extent to which the Greek scientific tradition itself was taken as a whole; and as a whole was criticized from various perspectives. But the concentrated critiques of the astronomical tradition, as was amply illustrated so far, must convince us of the need to consider this stage of Islamic astronomy as a new beginning for astronomy, in the sense that the need for a new astronomy had by then been clearly demonstrated.
In order to illustrate the fecundity of our new historiographic approach, and appreciate the repercussions of such critiques of the Greek scientific tradition as the ones that were leveled by someone like Ibn al-Haitham, and against the expectations of the classical narrative that marks this period after the eleventh century as a period of steady decline, I now turn to some later critiques in order to demonstrate the longevity of that tradition, and to indicate the direction it continued to follow. Rather than preserve the Greek scientific tradition, these later critiques, to which we now turn, illustrate the pervasiveness of the attacks against it.