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Alhacen's Theory of Visual Perception: A Critical Edition, With English Translation and Commentary, of the First Three Books of Alhacen's De aspectibus, the Medieval Latin Version of Ibn al-Haytham’s Kitab al-Manazir by Alhazen, edited, translated and introduction by A. Mark Smith, 2 volumes (Transactions American Philosophical Society vol. 91 parts 4 & 5) The importance of this thesis to the development of modern optics is well attested but the work has until now only been available in the Latin version with précis. The volume will be very welcome to scholars interested in the transmission of Arabic science to the medieval west and for historians of science and all who are curious how the science of optics developed in Europe.

From translators’ introduction: To evaluate Ibn al‑Haytham's achievement in optics objectively, or at least dispassionately, is no easy task, in great part because of the iconic stature he has assumed in the history of science. The result has been a tendency among scholars not only to emphasize the innovative character of his theory at both the conceptual and methodological level, but also to modernize that theory, or aspects of it, out of all proportion. Much therefore that has been claimed in Ibn al‑Haytham's behalf, though not necessarily untrue at the factual level, is nonetheless misleading at the interpretive level.

A particularly egregious example can be found in a recent issue of The New York Times Magazine devoted to "The Best Ideas, Stories and Inventions of the Last Thousand Years." According to the lead article in that issue, "Eyes Wide Open," by Richard Powers,361 Ibn al‑Haytham

deserves credit for the millenniums best idea, an idea whose significance is trumpeted in the article's header: "When an obscure Arab scientist solved the riddle of light, the universe no longer belonged to God." By Powers' reckoning, what Ibn al‑Haytham did (and herein lies the force of his idea) was usher in a new age of scientific empiricism, an age in which truth would be observationally, not theoretically, determined.

Or as Powers sums it up, "the idea of looking had begun to shake the foundations of authority as the basis of thought." How did Ion al-Haytham achieve this? By resolving "a scientific dispute [between

extramissionists and intromissionists] that had remained deadlocked for more than 800 years." He reached this resolution, Powers asserts, through a set of "remarkable observations," the simplest and most remarkable of which is as follows:

He invited observers to stare at the sun, which proved the point: when you looked at a sufficiently bright object, it burned the eye. He made no appeal to geometry or theoretical necessity. Instead, he demolished a whole mountain of systematic theory with a single appeal to data.

Light started outside the eye and reflected into it. No other explanation was consistent with the evidence.

Granted, this account has the twin virtues of simplicity and comprehensiveness, but these are its worst vices as well. Ibn al‑Haytham did point out that looking at the sun can impair vision (not, however, by burning the eye), but he cited this as evidence that light affects sight, not as disproof that the eye emits visual rays. Nor did any of the supposed beneficiaries of Ion al‑Haytham s insight mentioned by Powers (e.g., Roger Bacon, Witelo, William of Ockham, and Kepler) take it that way. Furthermore, much of the appeal of Ibn al­Haythani s account of vision lay not in its observational core but in its systematic features‑i.e., the idealized geometry of the eye, the selection of orthogonal rays by the lens, the sensitive function of the visual spirit, and so forth. It was on the basis of this appeal, in fact, that Ibn al‑Haytham assumed the very status of authority that Powers would have us believe he taught western thinkers to reject.

In all fairness, I should point out that Powers is a novelist by profession, not a historian, so I do not pretend that his assessment of Ibn al-Haytham represents the scholarly consensus. But it does, I think, represent an interpretive extreme based upon the assumption, held either explicitly or implicitly by many scholars, that Ibn al‑Haytham's Kitab alManazir was truly revolutionary in terms not only of its purport, but also of its import. On the one hand, in formulating his model of sight and light, Ibn al‑Haytham is supposed to have broken radically with the past. On the other, the appropriation of that model by Western thinkers is supposed to have led more‑or‑less inexorably to the development of modern optics. While these suppositions are not entirely groundless, they are problematic enough to warrant scrutiny before we assent to them. With that in mind, let us first address the issue of originality. Just how original was Ibn al‑Haytham s theory of vision and therefore how radically might he have broken with the past in formulating it?

Take the problem of radiation. At first blush, the differences between Ibn al‑Haytham s account of radiation and that of his visual‑ray antecedents appear to be so sharp as to be irreconcilable. Ibn al‑Haytham was unequivocal in his support of intromissionism, maintaining that visual contact between viewer and visible objects is established through the propagation of luminous color from those objects into the eye. Equally unequivocal in their support of extramissionism, his visual‑ray opponents maintained that the eye establishes visual contact with external objects by propagating visual flux to them. Yet, as we noted earlier, these two positions are far from irreconcilable. For one thing, despite his disagreement with the extramissionists over the direction and type of radiation, Ibn al‑Haytham preserved the basic analytic device of extramissionist optics, the visual cone, by transmuting it into a cone of vision. For another thing, according to the visual‑ray theorists, Ptolemy in particular, vision is completed only when the passion of coloring is conveyed back through the visual flux to the eye. Thus, for Ptolemy, as for Ibn al‑Haytham, visual perception ultimately depends upon the transmission of illuminated color from object to eye in the form of a cone. Indeed, Ibn al‑Haytham's refutation of visual radiation pivots upon this point. Even for the visual‑ray theorists, he contends, it is unnecessary to posit such radiation, because the complementary transmission of visual information back to the eye is perfectly sufficient. For yet another thing, Ibn al‑Haytham himself did not consider the two positions to be irreconcilable. He freely acknowledged that both the intromissionists ("philosophers") and extramissionists ("mathematicians") "have something true to say and that both opinions are correct and compatible." Since, however, "neither is wholly satisfactory without the other [to complement it], . . . vision [cannot] be properly accounted for without drawing upon what both have to say. 11370 Ibn al‑Haytham, of course, viewed his account as a proper melding of the two.

Still, to concede that the two models of radiation are equivalent at the mathematical level is not necessarily to deny their fundamental opposition at the physical level. Ibn al‑Haytham's conception of light and color is a good case in point. Virtually all of his classical predecessors treated light as a catalytic agent rather than as a direct object of sight. Its primary function was thus to render color visible, not to be seen in its own right. For Ibn al‑Haytham, on the other hand, light was per se visible, seen in its own right rather than through its effect on color. The contrast between these two conceptions of light could hardly be clearer at least in principle. In practice, though, it dwindles to virtual indistinction when we consider how Ibn al‑Haytham understood the relationship between light and color. That he viewed the two as ontologically distinct is beyond question, yet, by his own account, it is in the very nature of light to mingle with color. To be physically actualized, moreover, light must shine upon, or from, an opaque body; otherwise it cannot possibly be seen. But color is what renders such bodies visible. For all practical purposes, in fact, opacity is color. It therefore follows that light cannot manifest itself visibly unless it alloyed with color. Nor, for that matter, can color manifest itself visibly unless it is alloyed with light, because light gives color the capacity to be seen. Thus reduced to a theoretical abstraction, pure light becomes "visible" only by inference from its effect on embodied color. Effectively denied visibility in its own right, light thus assumes the role of catalytic agent in the visual process, its primary function being to render color visible. This is precisely the same function it has for Ibn al‑Haytham s classical antecedents.

Let us turn, finally, to the issue of methodology. One of the most persistent claims in behalf of Ibn al‑Haytham s originality is that, unlike his classical predecessors, he took an overwhelmingly empirical, or inductive, tack in analyzing light and vision. That this claim has a strong basis in fact needs no belaboring. We need only call to mind the plethora of experiments adduced throughout the Kitab al‑Manazir. Time and again Ibn al‑Haytham invites us to test his assertions by isolating the phenomena in question and submitting them to confirmation (or disconfirmation) according to carefully controlled circumstances. The appropriate apparatus is generally simple (e.g., a room with one window through which light shines on selected objects), but not always, as witness the elaborate device described in book 4 for verifying that light reflects at equal angles."" All things considered, then, we would be hard pressed to deny not only that Ibn al‑Haytham had strong empiricist leanings, but that his approach was essentially hypothetico‑deductive.

So much is beyond dispute, but the real issue is whether, in following his particular path of induction, Ibn al‑Haytham steered the science of optics in a new methodological direction. It is difficult to take this claim seriously in the face of Ptolemy's relatively heavy reliance upon empirical examples and experiment in the Optics. It is even more difficult in the face of Ibn al‑Haytham s intimate familiarity with that work. In at least two instances, key experiments described by Ibn al‑Haytham are strikingly similar to those outlined in Ptolemy's Optics, so similar, in fact, that there is little doubt that they are organically related. In addition, Ibn al‑Haytham and Ptolemy use the very same empirical examples (e.g., a spinning top or the oculogyral illusion) to illustrate the very same points. This is not to deny that there are differences, sometimes significant ones, between Ibn al‑Haytham s and Ptolemy's use of induction. Ibn al‑Haytham adduced many more experiments than Ptolemy, and in those cases where the experiments are parallel, Ibn al‑Haytham's are more elegant and elaborate than Ptolemy's. At bottom, though, these are differences in degree, not in kind. Ibn al‑Haytham may have been more inductive than Ptolemy at the quantitative level, but certainly not at the qualitative level.

By now it should be evident that, if analyzed in terms of its conceptual elements, idea by individual idea, Ibn al­Haytham's theory of light and vision reveals very little that is new or original 3'1 Indeed, far from breaking with the past, the Kitab al‑Manazir seems to be deeply imbedded in it. And so it is, especially when viewed in piecemeal fashion. But the Kitab al‑Mandzir is not a mere agglomeration of past ideas; it is a synthesis and should be evaluated as such. The originality of the Kitab al‑Mandzir thus lies in the way Ibn al‑Haytham reformulated and honed the ideas of the past and, on that basis, incorporated them into a seamless whole. The result is a grand reconciliation of nominally disparate, often conflicting, theoretical positions, a reconciliation moreover that comes across as perfectly natural and unforced. In a sense, then, Ibn alHaytham did transcend his past, not by overturning it but by reconfiguring and perfecting it. This is no small achievement, and the fact that is is not "revolutionary" in any meaningful way cannot detract from its underlying importance or ingenuity3'3

What, then, of the subsequent impact of Ibn al‑Haytham s visual theory in the Latin West; was it revolutionary? Again, I think the answer is a somewhat guarded "no." It could certainly be argued‑and indeed I have argued elsewhere‑that, with various Baconian elaborations, Ibn al‑Haytham s model served as a sort of paradigm of visual perception for later medieval and Renaissance thinkers .3" Yet this model did not constitute a paradigm in the strict Kuhnian sense, because, as we know from earlier discussion, it never fully supplanted the visualray alternative. Furthermore, unlike the archetypal Kuhnian paradigm, Ibn al‑Haytham s model of light and sight was welcomed by medieval Scholastic thinkers not in spite of its dissonance with their theoretical

preconceptions but precisely because of its consonance with them. In short, its acceptance required no suspension of deeply held belief. Not only did Ibn al‑Haytham not "shake the foundations of authority" in the Latin West (as Powers would have it); he went a long way toward establishing and bolstering them.

There is of course no gainsaying the depth and breadth to which Ibn al‑Haytham and his Perspectivist followers influenced medieval and Renaissance thought, not just in natural philosophy but also in theology and art. Nor is there gainsaying the fact that, without the theoretical

groundwork laid by Ibn al‑Haytham and his Perspectivist disciples, the revolution in optics inaugurated by Kepler and completed by Newton would have been, if not inconceivable, at least difficult to imagine. And herein lies the true significance of Ibn al‑Haytham s achievement: not that he overturned past optical tradition but that he brought it to logical perfection and, in doing so, inadvertently laid bare its vulnerability. For, as it turns out, his model of vision was flawed in at least two crucial respects. First, in supposing that the crystalline lens selects only orthogonal rays, Ibn al‑Haytham misconstrued the lens' real function, which is to bring all incoming radiation to focus on the retina. It took Kepler to correct this mistake at the beginning of the seventeenth century. Second, in supposing that light and color are ontologically distinct, Ibn al‑Haytham failed to realize that, in essence, light is color. It took Newton to correct this mistake toward the end of the seventeenth century.

To distill Ibn al‑Haytham's achievement down to the perpetuation of these two erroneous assumptions could easily be taken as a disparagement of that achievement; so central are these assumptions to his visual model that it would be no exaggeration to say that it stands or falls upon them. No doubt it is cold comfort to point out that, within their appropriate context, both assumptions are eminently reasonable, even necessary. Cold comfort, as well, to point out that the visual model arising from them is truly awesome in its coherence, comprehensiveness, and elegance. No matter the mitigation, the fact remains that, at least in retrospect, Ibn al‑Haytham was flat wrong. But if the history of science teaches us anything, it teaches us this: being wrong is not necessarily a bad thing. Quite the contrary, being wrong in the right way can be extraordinarily illuminating and, as such, can lead to extraordinarily fruitful consequences.  Looked at in this way, Ibn al‑Haytham's model of vision takes on an entirely new aspect, not just as a springboard but as an all‑important foil for the development of modern optics. On the one hand, in providing the key attack‑points for later thinkers, such as Kepler, Descartes, Huygens, and Newton, Ibn al­Haytham dictated the strategic lines of the ensuing battle. On the other hand, by sharpening the analytic tools of classical optics, Ibn al‑Haytham supplied his at­tackers with the weapons they needed to destroy his synthesis to its very foundations. The irony is inescapable. In giving seventeenth‑century theorists virtually everything they needed, Ibn al‑Haytham can be said to have fathered the optical revolution of the seventeenth century. Yet, in doing so, he was fated, like the titans of Greek mythology, to be undone by his thankless offspring.

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