[In the following essay, originally presented as a conference paper in 1973, Heinen considers al-Bīrūnī and al-Haytham, two Islamic scholars of the tenth century, paying specific attention to their methodology in the early development of scientific practices.]

Introduction

History of science is a relatively young academic discipline in modern institutions of learning; the individual sciences have grown hypertrophically, leaped forward to magnificent achievements, but also into ever greater independence and isolation. Man has been unable to catch up with these breath-taking advances, to integrate them into a unified human consciousness, the essential condition for an unrestricted and wholesome participation in all the macro- and microcosmic processes on which man's entire life depends. So in recent times ever more people, including specialized scientists, have become aware of the need to trace the historical development of the sciences.

Too often, though, this branch of history is guided by a rigidified modern conception of its specific object: science, not as one of man's activities only, but as the abstract, superhuman force, pictured as the ever triumphant conqueror of ignorance throughout the ages. The result is, as we all know too well, a flood of books and pamphlets that look so frighteningly alike: Lists of discoveries and inventions, ascribed to people of whom we seldom get to know much more than their names and such vital statistics as birth and death. These data are often enough the only thing that suggests a measure of historical awareness; for a man's scientific activities are carefully isolated from his other, supposedly less important, concerns: in contemporary and local society, religious movements, philosophical schools, etc.—No, if the history of science is to have a sounder basis than a dehumanized abstraction, it has to study science as the science of man, man the scientist being the center of its investigation, its fundamental reality.

A hopeful beginning, it seems to me, has been made by historians of science in antiquity and the Middle Ages, although they often do not guard carefully enough against the common trend of making rash comparisons with the achievements of the Renaissance or modern times, before the work of these earlier scientists has been duly studied and appreciated in itself. Thus A. C. Crombie's pioneering studies on Robert Grosseteste and his contemporaries¹ led to an increased interest in the methods and philosophical preconceptions of medieval scientists; not only their crystallized results, but even more their characteristic attitudes, and methodological choices that occasioned them, in short, their personal involvement as humane in scientific tasks, has become the more interesting 'object' of a history of science that is more truly a branch of history.

As far as the history of science in the Islamic countries is concerned, these broader, more human aspects of scientific development can still not be said to get the attention they deserve. In too many cases the basic texts are not even analysed and published yet, so that doing research in this area usually means that first the painstaking job of establishing the individual texts and their contributions has to be done. In some cases, however, thanks to the efforts of generations of devoted orientalists, this indispensable work has been completed already, or has at least progressed very far. And fortunately, this can be said of two of the greatest scientists in Islamic history: Al-Bīrūnī and Ibn al-Haytham. Thus the impatient student may be allowed to look ahead once and, relying on the ground-work laid by great scholars, gather some clues that cast light on the works of these two scientists, their origins and methods. This is done by way of comparison in order to give greater contrast to the characteristic features. In the case of Al-Bīrūnī and Ibn al-Haytham this procedure promises to be specially fruitful because they were contemporary Muslim scientists whose works covered so much the same ground that both men could be called "another Ptolemy"², but who lived so far apart from each other—and in such different states as Fatimid Egypt and Sunnī Ghaznah: that they apparently never came to know of each other's existence.

I have to admit I was quite pleased to find out that Eilhard Wiedemann, some forty years ago, already planned to write a comparative study of Ibn al-Haytham and Al-Bīrūnī³ this assured me that I was not after something unreasonable and useless, although I can hardly hope to provide a substitute for a study which this great pioneer of Islamic history of science had not had the time to carry out.

I. Ibn al-Haytham, the Ingenious Physicist

Although a distressingly big number of the works ascribed to Ibn al-Haytham and Al-Bīrūnī are lost without any hope of recovery, both men—and the destructive forces of the turbulent centuries that divide us from them—were good enough to leave us lists of their own writings and autobiographic reflections. Any study of their scientific works, and especially a comparative evaluation that tries to lay bare the roots and methodological orientations of their science, will therefore be able to start from their own explicit statements. Further details, however, will then have to be gathered from their other extant literary compositions; and only a careful reading of these works can show how far their scientific programs and methodological reflections have been carried out and where they have led.

To start with, the older one of the two men,⁴ Ibn al-Haytham is never satisfied to just state the facts as they present themselves; he always has to inquire into the causes, to ask the question 'why?'; thus he reflects on what made him a scientist and led him to his many discoveries, but this time he contents himself with an admission of ignorance:

I do not know how I attained to (what has been written down below) since the time of my youth. You may say, if you so wish, it was through a miraculous coincidence, an inspiration from God, a fit of madness, any other cause or whatever pleases you.⁵

He has no answer to this question, but he is fully convinced that nothing is more valuable and bringing man closer to God than the search for truth and knowledge.

This search—as Ibn al-Haytham sums up years of experience—has to be an independent one that endeavours to penetrate to the very roots and principles of knowledge and is not satisfied with what the traditional authorities have said about the matters under investigation. Considering the fact that steadfast loyalty to such authorities has determined intellectual history for thousands of years and has still a greater effect than we like to admit, Ibn-al-Haytham's critical attitude deserves all the more attention, especially since it follows from the mature experience of a great scholar. In his words:

Since my youth I have constantly reflected on the views of various people concerning their doctrinal convictions; every group of them tenaciously holds on to what it deems right according to the view that has taken form. I began to doubt about all this; for I became convinced that there is only one truth, and that its differing conceptions merely stem from the methods used in the investigation.⁶

A few words first about the critical part of this puzzling text: Sometimes it has been taken in isolation and interpreted in a restricted theological sense, as if Ibn al-Haytham had lost his faith in Islam or any existing religion and become a modern atheist. If his other writings are taken into consideration, it will appear that this interpretation is inspired by modern anti-religious dogma rather than by textual evidence. What Ibn al-Haytham in fact came to extol above everything else is a commitment to truth for its own sake, independent of school traditions and sectarian dogmatism; and although he has the highest regard for Aristotle and Ptolemy, whom he is happy to accept as master and guide, he nevertheless advocates the most vigorous criticism of any scientific writing. His program of methodical criticism should be at least as well known as that of the much later Descartes:—

Truth is sought for its own sake. And those who are engaged upon the quest for anything that is sought for its own sake are not interested in other things. Finding the truth is difficult, and the road to it is rough. For the truths are plunged in obscurity. It is natural to everyone to regard scientists favourably. Consequently, a person who studies their books, giving a free rein to his natural disposition and making it his object to understand what they say and to possess himself of what they put forward, comes (to consider) as truth the notions which they had in mind and the ends which they indicate. God, however, has not preserved the scientist from error and has not safeguarded science from shortcomings and faults. If this had been the case, scientists would not have disagreed upon any point of science, and their opinions upon any (question) concerning the truth of things would not have diverged. The real state of affairs is however quite different. Accordingly, it is not the person who studies the books of his predecessors and gives a free rein to his natural disposition to regard them favourably who is the (real) seeker after truth. But rather the person who is thinking about them is filled with doubts, who holds back with his judgement with respect to what he has understood of what they say, who follows proof and demonstration rather than the assertions of a man whose natural disposition is characterized by all kinds of defects and shortcomings. A person, who studies scientific books with a view to knowing the truth, ought to turn himself into a hostile critic of everything that he studies … he should criticize it from every point of view and in all its aspects. And while thus engaged in criticism he should also be suspicious of himself and not allow himself to be easy-going and indulgent with regard to (the object of his criticism). If he takes this course, the truth will be revealed to him and the flaws … in the writings of his predecessors will stand out clearly.⁷

The second, i.e. positive, part of Ibn al-Haytham's autobiographic statement quoted above takes us into the heart of his scientific originality.. "for I became convinced that there is only one truth, and that its differing conceptions merely stem from the methods used in the investigation." This unitary view of truth gives the characteristic vigour and direction to all scientific projects and achievements of Ibn al-Haytham; according to the first list of his writings, transmitted by Ibn Abī Usaybo'ah, he also wrote special treatises in support of this unitary view, however, they do not seem to have survived the destructions of history. Only their titles are still known:—

Treatise on the fact that there is only one method of attaining truth (nr. 25), and:

Treatise on the fact that there is only one proof, but that it is applied as a constructive one (sina') in geometrical questions, and as a dialectical one (kalāmi) in the physical and metaphysical questions (nr. 26).⁸

Since these general methodological treatises are lost to us, we have to turn to the introductions of the extant works, almost all of which contain a concise explanation of his methodology, and to these works themselves. What is most characteristic in Ibn al-Haytham's works is first the careful analysis of his object of investigation that allows him to distinguish its various scientific aspects, and then—with the simple consistency of the true genius—the combination of the appropriate methods for the study of these aspects;⁹ for the truth of the matter is only one, no matter how many different aspects, and correspondingly methods, the mind of the scientist produces. And it seems that Ibn al-Haytham gives preference to such objects and problems that more obviously permit or require the combined use of different scientific methods; his favourite one is doubtlessly light, and it is in optics where he made his most famous discoveries. He wrote a separate treatise on light in which he gives us a very clear and concise elucidation of his approach as scientist:

The discussion of what light is belongs to the natural sciences, and the discussion of the man of radiation of light reqires the help of the mathematical sciences because of the lines along which the rays of Light end. Similarly, the discussion of what the ray is belongs to the natural sciences, and the discussion of its form and shape belongs to the mathematical sciences. Similarly also the transparent bodies through which light passes. The discussion of what their transparency is belongs to the natural sciences, and the discussion of the manner how light extends through them belongs to the mathematical sciences. So it is necessary that the discussion of Light and ray and transparency belongs jointly to the natural sciences and the mathematical sciences.¹⁰

This method of jointly using the tools and arguments of different sciences for the same object of investigation is something surprisingly new, according to M. Schramm's thesis,¹¹ as new as Galileo's turn to modern physics, only—Ibn al-Haytham lived some 600 years before the great Italian scientist. For as the many examples of physico-mathematical research with frequent recourse to experimental observation in Ibn al-Haytham's various optical works show, his science is already—at least in this particular field—that systematic use of mathematical functions for the description of physical processes which, since Galileo has marked the swift progress of modern physics. Surely, he still sounds quite Aristotelian when he speaks of light as a substantial form; but for him the intensity of this form can mathematically be described as a function of the angle of refraction, for instance.

The consequences of this new approach to scientific problems are naturally quite far-reaching. Physics deals no longer merely with abstract principles, aiming at the philosophical description of the natures and qualities of the various substances, but with the operational effects of these things that can be measured and mathematically formulated in functions. And since Ibn al-Haytham's primary object of physical research is much a universal phenomenon as light, being part of the sublunar as well as of the supralunar world, his new physics extends quite logically to the planets and stars, while with Galileo the laws of earthly mechanics are simply projected on the movements of the heavenly bodies. Astronomy, consequently, is in Ibn al-Haytham's view of the sciences not limited to purely mathematical circles and spheres, invented to "save the phenomena", but it deals with the real movements of the heavenly bodies and therefore also real spheres.

The consequences for mathematics, especially geometry, are even more staggering, as appears principally in his short treatise on space, a remarkable elaboration of the basic Aristotelian conception, but far beyond Aristotle.¹² Space is for Ibn al-Haytham no longer identical with the surface of the surrounding body, but it is an imaginary three-dimensional entity that consists of the dimensions extending between the opposite points of the area surrounding an imaginary vacuum. However, it does not exist in an absolute sense, as he seems to have clarified in a special treatise of which we only have the title: "Treatise on the non-existence of an empty or a fully occupied space beyond the heavens".¹³ In any case, spatial extension, the very basis of geometry, is seen in correlation with the physical interaction of bodies to such an extent that the path of light as the straightest possible line is actually proven by way of experiment. Surely, this particular experiment is not very fanciful at all. A beam of light is simply collated with a straight rod and declared straight because no divergence can be perceived. But it is remarkable that early in the Middle Ages the straightness of light beams is neither simply taken over from the long tradition of optics nor deduced with the help of a priori principles, but is seen as merely the physical agreement with a standard taken from ordinary sense perception. Is it too much to say that quite modern developments in science are fore-shadowed in this unusual scientific attitude of Ibn al-Haytham?

II. Al-Bīrūnī, the Great Mathematician

This mathematical point may lead us over to the discussion of the scientific methodology of al-Bīrūnī, surely one of the greatest mathematicians of the Middle Ages. To consider his work after that of Ibn al-Haytham will make it easier for us to select the essential features of his approach as a scientist which otherwise tend to get lost in the very extensive and variegated investigations, calculations and historical accounts that make up his major writings. And al-Bīrūnī has not reflected so intensively on his own method as did his great contemporary Ibn al-Haytham.

The contrast between the two contemporary scientists is quite obvious from the beginning: While Ibn al-Haytham, as he himself informs us, endeavoured systematically to absorb and further develop the Aristotelian treasure of knowledge, scientific work seems to have come quite spontaneously to al-Bīrūnī, almost as the natural outflow of his rich personality. All his books bear this mark of effortless, and sometimes playful, ingenuity; one does not mind his numerous digressions at all, because whatever he writes is highly interesting and—inspite of his mathematically concise language—extremely informative. It is curious how the great al-Bīrūnī in his old age reflects on his awakening interest in science early in his childhood, namely in the preface of his last book, the Kitāb al-Saydānāh, which he apparently left unfinished:¹⁴ A foreigner, who appears to have been a Byzantine merchant, travelled through young Abū Raihān's hometown and gave the child an opportunity to inquire about foreign names for the familiar objects of his environment. This little incident is characteristic of large sections of the later "master's" scientific publications: Precise descriptions of corresponding systems used in different nations for astronomical or astrological calculations, fixation of calendars, geographical determinations, mineralogical and pharmacological handbooks, philosophical speculations, etc. Their scientific value rests less in forward-looking inventiveness than in their exact and objective manner of description and the enormous amount of information they contain.

For al-Bīrūnī all scientific research seems to hinge on one or the other branch of mathematics, and mathematics itself is rooted in man's nature:

Counting is innate to man. The measure of a thing becomes known by its being compared with another thing which belongs to the same species and is assumed as a unit by general consent. Thereby the difference between the object and this standard becomes known¹⁵

This natural drive of man to count the objects around him, and to establish a quantitative correlation among them, accounts for most of al-Bīrūnī's scientific work, although he himself also repeatedly stresses usefulness of knowledge as an important motive for his own research and the promotion of science in general. Since this usefulness is, in his view, primarily the one to religious man, as he emphasizes even in such a technical book as the Kitāb Tahdīd al-Amakin, science assumes a certain religious overtone:

We look around and we see that man's efforts are directed only towards earning a living, and for this purpose he endures hardships and fears, though he needs his food only once or twice a day for his life in this world. But he pretends ignorance and neglects what he must not fail to do for his soul in the hereafter, five times in every day and night, thinking that his ignorance is a valid excuse, though he has the opportunity and the power to know it (what is good for his soul).

The Jews also need a direction, because they turn in their prayers to the Temple in Jerusalem which is of known longitude and latitude …—The Christians need the (direction of) true east because their elders, whom they call fathers, prescribed to them that they should turn to Paradise in their prayers.…¹⁶

This whole perspective is strikingly different from Ibn al-Haytham's science for science's sake; but this text should not mislead us either, as if truth in itself was not beautiful enough for al-Bīrūnī, "for (as he says in the same book) the intrinsic worth of a thing is different from the casual material benefit that it brings in its train".¹⁷ And it is knowledge in itself that makes a true human being of man:

… It is knowledge, in general, which is pursued solely by man, and which is pursued for the sake of knowledge itself, because its acquisition is truly delightful, and is unlike the pleasures desirable from other pursuits.¹⁸

Furthermore, al-Bīrūnī's attitude to former scientists and their schools, as well as to religious and philosophical traditions, differs quite markedly from that of Ibn al-Haytham who shows himself impressed by hardly anyone besides Aristotle and Ptolemy, and even in regard to them grows increasingly independent. Al-Bīrūnī, on the contrary, does not hesitate to declare:

I must assay all aspects of this statement, because I do not refuse to accept the truth from any source, wherever I can find it.¹⁹

In light of this statement it cannot surprise that his writings abound with quotations from and references to the works of other scholars. However, he does not adhere to one master's school, to the extent to which Ibn al-Haytham always remains an Aristotelian. He has words of high praise for al-Rāzī, but he explicitly dissociates himself from the school of this famous Persian physician and philosopher.²⁰ But the many quotations from Plato seem to suggest that he had a definite preference for the Platonic tradition.

We saw above how Ibn al-Haytham's strong conviction of truth being only one, inspite of the methodological differences, led him to a new and most fruitful combination of mathematics and physics. Al-Bīrūnī, for his part, keeps the different sciences clearly distinct and he considers it possible and preferable to pursue them one at a time. This might seem to be the natual thing to do, but it does not give consistency to his whole scientific system, as the comparison with that of Ibn al-Haytham shows. Take, for instance, his extensive use of mathematical calculations in their application to the physical realities of the universe: Infinity appears to be their sole logical termination, nevertheless, al-Bīrūnī draws a certain—or rather uncertain—line somewhere, as a sarcastic remark in his India shows:

We on our part found it already troublesome to enumerate all the seven seas, together with the seven earths, and now this author thinks he can make the subject more easy and pleasant to us by inventing some more earths below those already enumerated by ourselves.²¹

Enumeration and mathematical calculation must come to an end at some point; but where exactly? In view of Ibn al-Haytham's fruitful combination of mathematics and physics, and the later development of science, it must look like a tragic omission in al-Bīrūnī's scientific system that he was clearly aware of the problem, but did nothing to solve it satisfactorily. Since he does not modify his scientific approach following the admission of physical limits at which man's counting and measuring comes to an end, it must appear as a mere theological or philosophical restriction that for him man's knowledge cannot reach out indefinitely into space and time, but goes only as far as sense perception. Thus he approvingly quotes from the followers of the Hindu philosopher Aryabhata:

It is sufficient for us to know the space which is reached by the solar rays. We do not want the space which is not reached by the solar rays, though it be in itself of an enormous extent. That which is not reached by the rays is not reached by the perception of the senses, and that which is not reached by perception is not knowable.²²

And concerning time, al-Bīrūnī himself makes a similar restricting statement:

We do not know of the conditions of creation, except what is observed in its colossal and minute monuments which were formed over long periods of time, for example, the high mountains which are composed of soft fragments of rocks, of different colors, combined with clay and sand which solidified over their surfaces. A thoughtful study of this matter will reveal that the fragments and pebbles are stones which were tom from the mountains by internal splitting and by external collision … All those changes are necessarily of long duration, and their causes are of unknown nature.²³

The last sentence must baffle the critical reader; for it is hard to understand how the great astronomer and scientist al-Bīrūnī can so offhandedly push aside any investigation into the proximate physical causes of such earthly changes as those of erosion. The most likely explanation seems to be that his Platonic approach to reality, or his high appreciation of mathematical exactitude that simply could not be found in the sublunar world to the same extent as in the movements of the heavenly spheres, stood in the way, preventing him from aspiring after a complete scientific explanation of all the natural phenomena.

It should not be forgotten that al-Bīrūnī's achievements as one of the greatest mathematicians and astronomers of the Middle Ages are so outstanding in themselves that they alone lift him above all his contemporaries. But they probably would have been even more fruitful had he not, because of a particular philosophical preconception, been satisfied to merely negatively limit his research to the physically, i.e. through the rays of the sun, accessible world, and, instead, drawn the positive conclusions for all possible scientific knowledge of our world; after all, Ibn al-Haytham did precisely this, when he realized that even our sense organs are parts of the physical world, and that therefore all human science has to be composed of physics and mathematics, or sense phenomena as matter and mental structures as form.²⁴ By his scientific endeavours man participates in the processes of this world, he does not merely lay his mathematical co-ordinates over it, the work in which al-Bīrūnī seems to have found the greatest pleasure.

Surely, al-Bīrūnī was aware of the necessary correlation between the observer's logical and mathematical systems, on the one side, and the dynamic interaction of physical bodies, on the other; but that does not occasion him to engage in any elaborate studies of these dynamic aspects, not even to the extent of laying a firm basis for his own observations and measurements. As we saw above, in the case of Ibn al-Haytham it was the inquiry into the nature of light that led him to the discovery of a new scientific method, so extremely fruitful in modern physics; al-Bīrūnī, for his part, was satisfied with the mere question whether light was material or immaterial, and his Optics seems to have followed the traditional mathematical line.²⁵

In addition to light, there were, of course, innumerable other natural phenomena and processes that could have served as stimuli for al-Bīrūnī to make his own, doubtlessly valuable, contribution to physical science, the most likely ones being the physical effects of the heavenly bodies on each other, and especially on all events here on earth. But, inspite of his many, most exact astronomical observations and mathematical calculations, carried out with no less effort and vigour in astrology than in astronomy, he merely enumerates the various physical effects of the moon, for instance, on earthly substances and events, almost in the same fashion as Ibn al-Haytham, in the beginning of his Optics, reviews the variegated effects of light in our environment. However, we look in vain for a detailed examination based on experiments and a mathematical description of the processes by means of functional correlations. Consider, for instance, the following text from his India, to which many others from his Astrology could be added:

That the moon has certain effects on moist substances, that they are apparently subject to her influences, that, for instance, increase and decrease in ebb and flow develop periodically and parallel with the moon's phases, all this is well known to the inhabitants of seashores and seafaring people. Likewise physicians are well aware that she affects the humors of sick people, and that the fever-days revolve parallel with the moon's course. Physical scholars know that the life of animals and plants depends upon the moon, and experimentalists know that she influences marrow and brain, eggs and the sediments of wine in casks and jugs, that she excites the minds of people who sleep in full moonlight, and that she affects linen clothes which are exposed to it. Peasants know how the moon acts upon fields of cucumbers, melons, cotton, &c., and even make the times for the various kinds of sowing, planting, and grafting, and for the covering of the cattle depend upon the course of the moon. Lastly, astronomers know that meteorologic occurrences depend upon the various phases through which the moon passes in her revolutions.²⁶

Nobody would deny that this list includes many phenomena that should challenge the curiosity of any scientist, as light did in the case of Ibn al-Haytham. A precise casual analysis, based on extended observations and carefully planned experiments, could enable him to give a more exact and detailed description of the processes involved, and with the necessary mathematical functions he could formulate the universal laws that would make the different phases of these processes truly comprehensible. But nowhere in al-Bīrūnī's voluminous works do we find traces of such physical investigations. They would seem to be the natural prerequisites of his numerous and most intricate astrological calculations, still they do not seem to have ever been made by al-Bīrūnī. The question, whether he "believed" in astrology or not, has often been discussed; Sachau's view, namely, that he must have had some belief in it, considering all the material he published on it, might be modified in the sense that he "believed" in it insofar as it gave him plenty of opportunities to practise his mathematical skills, but that he never cared much for the physical effects ascribed to the planets and stars, since he not even cared for the ordinary casual interactions between bodies in the sublunar or supralunar world.

By way of conclusion, the comparison between two contemporary scientists can only underscore the importance of methodological choices for the development of science and man's total relationship to his whole physical nature and the universe around him. For, al-Bīrūnī and Ibn al-Haytham can be said to be comparable to each other as are the scholastics of the 14th century, with their highly developed mathematics, with Galileo, the founder of modern physics—i.e. the difference is one of three centuries if the development of science is considered.

Notes

¹ Crombie, A. C.; Robert Grosseteste and the Origins of Experimental Science, 1100 - 1700; Oxford, 1953.

² The reason might be that both men wrote commentaries on the Almagest.

³ See: al-Bīrūnī: The Book of Instruction in the Elements of the Art of Astrology; transl. by R. Ramsay Wright; London, 1934; Preface, II.

⁴ Ibn al-Haytham was born in 965 A.D.; cf. E. Wiedemann: "Ibn al-Haitam, ein arabischer Gelehrter"; in: Festschrift J. Rosenthal; Leipzig, 1906; p. 151.

⁵1bid, p. 157 (My own translation).

⁶Ibid, p. 156 f

⁷ S. Pines: "Ibn al-Haytham's Critique of Ptolemy"; in: Actes du Xe Congres internationale d'histoire des sciences; Ithaca, 1962 (Paris, 1964), I., p. 547 f.

⁸ E. Wiedemann: "Ibn al Haitam, ein arabischer Gelehrter"; op. cit., p. 165.

⁹ Cf. E. Wiedemanm: Theorie des Regenbogens von Ibn al Hatam; in: Aufsatze zur arabischen Wissenschaflsgeschichte; Hildesheim—New York, 1970; Bd. H, p. 71.

¹⁰ Ibn al-Haytham: M (or Qawl) f al-Daw; nr. 2 in: Majmu' al-Rasa'il; Hyderabad, 1357 A.H. (1938); p. 2 (My own translation). In this context I would like to thank Prof. A. I. Sabra for most helpful suggestions.

¹¹ See: M. Schramm: Ibn al-Haytham Weg Zur Physik; Wiesbaden, 1963.

¹² See: E. Wiedemann: Kleinere Arbeiten von Ibn al Haitam. 1. Eine philosophische studie von Ibn al Haitam Uber den Ort; in: Aufsatze, op. cit., Bd. I., p. 525, esp. note 1 (by Horten).

¹³ E. Wiedemann: lbn al Haitam, ein arabischer Gelehrter; op. cit., p. 165.

¹⁴ Cf. M. Meyerhof: Al-Bīrūnī, Das Vorwort zur Drogenkunde des Beruni. ed. and transl.; Berlin, 1932.

¹⁵ E. C. Sachau: Alberūnī&ni's India; London, 1910; Delhi, 1964 (Reprint), I., p. 160.

¹⁶ Al-Bīrūnī: The Determination of the Coordinates of Positions for the Correction of Distances between Cities; transl. by Jamil Ali; Beirut, 1967; p. 175; cf. p. 3.

¹⁷Ibid., p. 8.

⁸Ibid., p. 2.

¹⁹Ibid., p. 79.

²⁰ See: E. Wiedemann: Uber al-Bīrūnīal B r n und seine Schriften; in: Aufsatze, op. cit., p. 487 (Bd. II).

²¹ E. C. Sachau: Alberūnīm's India; op. cit., I., p. 237.

²²lbid, p. 225; cf p. 227.

¹³ Al-Bīrūnī: The Determination; op. cit., p. 16 f.

²⁴ Cf. E. Wiedemann: Ibn al Haitam, ein arabischer Gelehrter; op. cit. P. Lf&.

²⁵ Nothing definite can be said, though, because it seems to be lost.

²⁶ E. C. Sachau: Alberiini's India; op. cit., I., p. 346 f.