It was only after a residence in Egypt of more than twenty years that Maimuni obtained an appointment as physician at the court of Saladin; up to that time he had acquired only a slight practice. He was not Saladin's physician in ordinary, for the Sultan, on account of the constant wars with the adherents of Nureddin and with the Christians, could not visit his capital for a long time. But the favor of the noble vizir, the wise and mighty Alfadhel, who was also a great promoter of learning, and of whom a contemporary said, "he was entirely head and heart," was of as much value as the distinguished recognition of the sovereign. Alfadhel caused Maimuni to be placed on the list of physicians, settled upon him a yearly salary, and loaded him with favors. Inspired by his example, the great men of the country who lived in Cairo likewise bestowed upon him their patronage, so that Maimuni's time was so fully occupied that he was obliged to neglect his studies. Maimuni was indebted for his elevation more to his medical learning than to his skill as a physician; for he pursued this profession as a learned science, and prescribed no recipe for whose efficacy he could not cite the judgment of medical authorities. He treated the facts of scientific medicine in the same spirit as he had treated the Talmud. In this manner he elaborated the writings of Galen, the medical oracle in the Middle Ages; he abridged and arranged them, without permitting himself to deviate from the original in the slightest particular. The same character is borne also by his medical aphorisms, which are nothing further than extracts from and classifications of older theories. In spite of his almost absolute lack of originality in the province of medicine, Maimuni nevertheless enjoyed a wide reputation as a medical author. The celebrated Mahometan physician and theologian, Abdel-latif, of Bagdad, who enjoyed the favor of Saladin in a high degree, confessed that his wish to visit Cairo was prompted by the desire to make the acquaintance of three men, among whom was Musa ben Maimun. The poet and kadhi, Alsaid Ibn-Sina Almulk, sang of Maimuni's greatness as a physician in ecstatic verse:
"Galen's art heals only the body,
But Abu-Amran's (Maimuni's) the body and soul.
With his wisdom he could heal the sickness of ignorance.
If the moon would submit to his art,
He would deliver her of her spots at the time of full moon,
Cure her of her periodic defects,
And at the time of her conjunction save her from waning."
Maimuni's reputation was so great that the English king, Richard Cœur-de-Lion, the soul of the third crusade, wanted to appoint him his physician in ordinary, but Maimuni refused the offer.
His patron, the chief judge and vizir Alfadhel, acquitted him at about this time of a grave charge, for which, under a less mild Mahometan, or even a Christian judge, he would have incurred the penalty of death. The same Abulalarab Ibn-Moïsha who had befriended Maimuni in Fez, had come from Maghreb to Egypt, and when he saw Maimuni, whom he had known as a Mahometan, at the head of the Jewish community as spiritual chief, he appeared against him as an accuser, and averred that Maimuni had for a long time professed the religion of Islam, and consequently ought to be punished as a renegade. Alfadhel, before whose tribunal the accusation was preferred, decided rightly that the compulsory adoption of a creed could have no value, and, therefore, could involve no penalties (about 1187). In consequence of his favor with the vizir, Maimuni was appointed supreme head of all the Egyptian congregations, and this dignity descended in his family from father to son and grandson. It is certain that Maimuni drew no salary for this office, for nothing appeared to him more discreditable and sinful than to receive payment for the discharge of spiritual duties, or to degrade knowledge into a money-making business. He sought this prominent position not for himself, but for the sake of his co-religionists, in order to save them from injustice. It was through him that the heavy yoke of persecution was removed from the congregation of Yemen. When Saladin had once more wrested Jerusalem from the hands of the Christians, who had held it for nearly a century, he allowed the Jews to settle in the city of their fathers (October, 1187). And from all sides there came devoted sons to visit their mourning and forsaken mother. Possibly Maimuni was not unconnected with this act of noble-minded tolerance. Lastly, he endeavored to obtain for his brethren in faith precedence in the state over the Karaites, and gradually to oust the latter from their favorable position at court, so that many of them reverted to Rabbanism. This was accounted to Maimuni as a most meritorious deed in his time.
The higher Maimuni advanced in the esteem of his contemporaries, the more his extraordinary ability was acknowledged, and the louder his fame resounded, the more did the arrogant Samuel ben Ali, of Bagdad, feel himself belittled, and the more did he become filled with envy. Samuel accordingly took every opportunity to depreciate Maimuni's merit, and rob him of his fame. Samuel and his friends whispered to one another that Maimuni was by no means a strictly religious Jew, nor a true follower of the Talmud, and they spread many calumnies about him. Some mistakes which he had made in his youthful work, the Mishna Commentary, were used by these malevolent people with a view to brand him as ignorant of the Talmud, and without claim to authority in this province. Their idea of religion, as Maimuni said of them, consisted in guarding against the violation of precepts; but according to their view, good morals, humility, merely human virtues, in short, do not belong to religion. As the seed which Maimuni had scattered began to bear fruit, Samuel ben Ali and his allies took advantage thereof to lower the author in the eyes of his contemporaries.
In Damascus and Yemen there appeared religious teachers, who drew from Maimuni's writings logical conclusions which he himself did not care to deduce. As he strongly affirmed, and repeatedly insisted, that by the immortality of the soul a purely spiritual existence in another world was to be understood, whereas he passed over the resurrection of the dead as of only secondary importance, his disciples concluded that he was not thoroughly convinced of the resurrection, and forthwith began to teach that after death the body sinks into dissolution and decay, and that only the soul becomes elevated to a purely spiritual life. This liberal view clashed with explicit declarations in the Talmud, and consequently aroused general opposition. Samuel ben Ali was requested by some one in Yemen to give his opinion on this question of the belief in the resurrection. Samuel wrote a whole treatise upon it, with philosophical flourishes, in order to appear a worthy rival of Maimuni, and seized the opportunity of criticising the latter's writings, hoping to heighten the effect of the criticism by according partial praise to Maimuni. On another occasion, Samuel ben Ali directed a letter to Maimuni, in which, amid much flattery and fawning, he reproached him with having committed an error in interpreting the Talmud, which could scarcely have been made by a beginner, kindly adding that Maimuni must not fret himself about it. At the same time, he did not forget to promise graciously to take him under his protection against the congregation in Yemen. Maimuni replied with a heated letter, in which he showed his malicious opponent that it was he who had erred in the deeper conception of the Talmud. He also touched upon the secret attacks made against his great work from this quarter, some asserting that the book contained mistakes, others that it was superfluous, others, again, that it was dangerous. "You seem," Maimuni observed to him, "to reckon me among those who are sensitive to every word of blame. You make a mistake. God has protected me against this weakness, and I protest to you, in His name, that if the most insignificant scholar, whether friend or foe, would point out to me an error, I would be grateful for the correction and instruction." Although Samuel ben Ali was readily refuted by Maimuni, he still continued to spread the report that the latter was no Talmudist, and that his codex did not deserve the respect which it enjoyed. From another side, from Haleb, Mar Sacharya, a man of limited range of vision, and with a superficial knowledge of the Talmud, thinking himself eclipsed by Maimuni's pupil, Joseph Ibn-Aknin, worked with equal hostility against master and disciple. But, as the sage of Fostat had warm and disinterested adherents everywhere, Samuel ben Ali and his ally of Haleb were constrained to act cautiously. They organized an intrigue against him, into which they drew one of the two Exilarchs. Towards this cabal, Maimuni assumed an attitude of contemptuous indifference and unconcern, which altogether disarmed his opponents.
In spite of his collisions with the party of Samuel ben Ali, and his prodigious activity as a physician, which scarcely gave him time for study, he completed his religious philosophical work, "Guide of the Perplexed" (Moreh Nebuchim, Dalalat al Haïrin) in about 1190. This treatise became of extraordinary importance, not only for Judaism, but for the history of philosophy in the Middle Ages generally. Maimuni appears at the summit of his intellectual power in this work, and it contains the vindication of his profoundest convictions. The questions which the human mind starts ever anew, about the existence of a higher world, the destiny of our being, and the imperfection and evil of the earthly world, Maimuni sought to answer in a manner which was at that time considered convincing. The doubts which the thinking Jew may conceive of the truth of his hereditary religion, he endeavored to remove in a persuasive manner. He, whose thoughts were ever directed to the loftiest subjects, could with justice assume the character of guide to the perplexed and wavering. The external form of this epoch-making work would make it appear that the author had elaborated, for his favorite disciple, Joseph Ibn-Aknin, of Fez, separate treatises on important points which had disquieted and tortured the latter. But it was actually dictated by the desire to express clearly his philosophical conception of the world, and his views of the place which Judaism finds in it, and thoroughly to analyze their mutual relation.
Maimuni was, on the one hand, firmly convinced of the truth of the Aristotelian philosophy, as the Mahometan philosopher Ibn-Sina and others had formulated it. On the other hand, Judaism was to him a body of truths not less irrefragable. Both seemed to him to have the same conclusion and a common aim. Philosophy recognizes as the principal of all essences one indivisible God, the governor of the world. Judaism likewise teaches with emphatic asseveration the unity of God, and abhors nothing more thoroughly than polytheism. Metaphysics knows no higher aim for man than that he should perfect himself intellectually, and work his way up to the highest knowledge. Judaism also, even Talmudical Judaism, places understanding and knowledge, the understanding of God, at the head of its precepts. If the truth which the human mind in the fulness of its power evolves from itself, and the revelation which the Deity vouchsafed to the Israelitish nation on Sinai, resemble each other in beginning and end, then their separate parts must correspond with each other, and be as one and the same truth, arrived at in different ways. Judaism cannot be in contradiction with philosophy, as both are emanations from the divine spirit. The truth which God has revealed must also agree with that which lies in the human reason, since the latter is a power originating from God, and similarly all truths which metaphysical thinking can bring to light must exist in the revelation—that is, in Judaism. Hence, Maimuni believed that originally, besides the written revelation in the Pentateuch, there were also communicated to the greatest of prophets oral doctrines of a philosophical character, which were transmitted by tradition to posterity, and which were lost only in consequence of the troubles and afflictions which the Israelites experienced in the course of ages. Traces of this old Israelitish wisdom are found, according to Maimuni, in the scattered utterances of the prophets, and in the reflections of the Agada. When, therefore, the thinking Jew borrows the truths of Greek philosophy, and adopts the theories of Plato and Aristotle, they are not altogether strange elements to him, but only a reminder of his own forgotten treasure.
The whole universe, which must be considered as a single organic whole, consisting of spheres suspended over one another working in harmony, is nothing more than the realized thoughts of God, or rather than the ideas of God ever tending to realization. He continually imparts to it new forms and shapes, and implants order and regularity in the world. Everything is arranged therein in accordance with a final purpose. The Greek philosophy, it is true, assumes that the universe shares in the eternity of God; but it can neither irrefutably prove the eternity of the world, nor remove any of the difficulties which oppose the acceptation of the original existence of the universe. The doctrine of Judaism is much more reasonable, that the world had a positive beginning, and that time itself, which, indeed, is a form of the world and its motion, is not without beginning, but was called into being by the determining will of God.