2. In Cairo, at the beginning of the 11th century, we come upon one of the most considerable mathematicians and physicists in all the Middle Ages, Abu Ali Mohammed ibn al-Hasan ibn al-Haitham (Alhazen). He had formerly been a government-official in Basra, his native town. Confiding too much in the practical value of his mathematical knowledge, he imagined that he could regulate the inundations of the Nile; but having been summoned on that account by the Caliph al-Hakim, he became aware, soon after his arrival, of the futility of his efforts. Thereupon he fell into disgrace as a public official, and went into hiding till the Caliph’s death, in 1021. From that time he devoted himself to literary and scientific work, up to his own death, in 1038. His chief strength is shown in mathematics and its practical application; but he also devoted great attention to the writings of Galen and Aristotle, nor did he confine that attention to the physical treatises. By his own confession he had, in a spirit of doubt about everything, been engaged, from his youth up, in considering the various views and doctrines of men, until he came to recognize in all of them more or less successful attempts to approximate to the truth. Moreover truth for him was only that which was presented as material for the faculties of sense-perception, and which received its form from the understanding, being thus the logically-elaborated perception. To seek such truth was his aim in the study of philosophy. In his view philosophy should be the basis of all the sciences. He found it in the writings of Aristotle, inasmuch as that sage had best understood how to knit sense-perception into a coherent whole with rational knowledge. With eagerness therefore he studied and illustrated Aristotle’s works, for the use and profit of mankind, as well as to exercise his own intellect and provide a treasure and consolation for his old age. Of these labours, however, nothing seems to have been preserved for us. The most important of Ibn al-Haitham’s writings is the “Optics”, which has come down to us in a Latin translation and redaction. In it he shows himself to be an acute mathematical thinker, always taking pains with the analysis of hypotheses and of the actual examples. A Western, belonging to the 13th century (Vitello), was able to give a more methodical account of the whole subject; but yet in keenness of observation on specific points, Ibn al-Haitham may be reckoned his superior. 3. Ibn al-Haitham’s thinking is expressed in quite a mathematical style. The Substance of a body consists, according to him, of the sum of its essential attributes, just as a whole is equal to the sum of its parts, and a concept to the sum of its marks.