Otherwise known as al-Risala al-Kamiliyya fil-Sira al-Nabawiyya.
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The English translation is literal in some parts, abbreviated in others. It was carried out by Meyerhof and Schacht. The Arabic is in the second half of the PDF.
Please download and reupload on other servers because I don't know how long that link will be valid.
Westerners call it one the earliest examples of a science-fiction novel or story.
It's a short story about a man who is spontaneously generated in a cave on a remote desert island and how he reasons out conclusions about the world around him. He's then picked up by another ship and taken to civilization where he continues his conclusions. He deduces most of the major points of 'aqeedah and Prophet Muhammad's (saw) history and mission (without the aid of Scripture), and then goes on to deduce future events that had not yet happened, including some of the major signs of the Day of Judgement (explaining them from a purely rational/scientific standpoint). He also covers some events of the time in which Ibn al-Nafis lived (the attack by the Mongols and the Mamelukes' defense).
So it starts off concerned with philosophy (including some "sci-fi" concerning the events which led to the protagonist's spontaneous generation), then after he's picked up by humans it turns towards theological issues. It ends with "science-fiction", an apocalyptic doomsday scenario (that concurs with Islamic theology/eschatology).
It's considered a Maturidi (well, Ash'ari but Ibn al-Nafis is working almost purely from the Maturidi view here) response to the earlier Philosophus Autodidactus (Hayy ibn Yaqdhan) by Ibn Tufail. That was a more popular work espousing some basic Mutazilah philosophy mixed with Ash'ari theology and Sufism (it was a "thought experiment" response to Al-Ghazali's Incoherence of the Philosophers). That work doesn't get into theology so much (because it was a popular opinion among those with Mutazilah reasoning that reason could not substantiate revelation), but it does cover some spiritual principles (strongly influenced by Sufism, so it's not like some sort of Mutazilah/Rationalist propaganda) as a sort of "replacement" for reason (but to those who don't believe in spirituality, mysticism is just another aspect of self-introspection and insight, and thus a part of the 'aql / Reason). Using his senses (hassi or hissi) and reason ('aql), the protagonist of Ibn Tufail's story discovers God, morality, and science. Regarding the latter, it mostly deals with the philosophy of empiricism (which is the basis for scientific inquiry and in which Ibn Sina, working from the philosophy of Aristotle, made a lot of developments... it's a philosophy that is most obviously shared by Ahlus Sunnah wal Jamaat in case anyone's wondering).
What basically happened is that the Mutazilah emphasized reason and sensory experience (the individual) as a source of knowledge but also claimed that reason could not prove certain aspects of revelation. The debates between Al-Ghazali and Ibn Rushd were basically regarding the primacy of reason versus revelation. The intention behind this from the perspective of the orthodox theologians was that reason is subject to error so it cannot be held equivalent to or higher than revelation. So, naturally, Al-Ghazali had to take a very skeptical stance towards all aspects of rationalism. However, this exercise in and of itself showed that reason and rationalism could back up revelation but was subject to the errors of the mind wielding them. In other words, the Ash'ari and Maturidi schools (especially the Maturidi) uphold the power and scope of human reason ('aql) and sensory experience (hissi) but just do not use it as a source of knowledge equivalent to revelation (or even other true narratives). This is a significant epistemological difference because it's basically a different view of humanity and emphasizes the social nature of humans as opposed to the individual, as the Mutazilah philosophy did. In fact, the philosophy of the Ash'ari/Maturidi thinkers in the natural sciences was superior to the Mutazilah and it was a natural evolution of thought. Just as how primitive Greek thought gave way to the first generations of Muslim thinkers. Imam Ash'ari himself symbolized this transfer of power by the fact of his Mutazilah upbringing, then deciding it wasn't cutting it anymore.
Ibn Sina thought that reason could not prove the idea of bodily resurrection.This work, based on Ibn al-Nafis' own considerable experience as a physician and scientist, showed otherwise. He showed that reason and empirical scientific inquiry could back up revelation. He himself made significant contributions to human knowledge of anatomy and physiology, specifically pulmonary circulation (no doubt because many people feel that the "ruh" is related to the blood's circulation... without the circulation, death occurs). He also covers topics resembling modern notions of abiogenesis (the spontaneous generation of the protagonist in the cave from clay and water) and his defense of "bodily resurrection" with science, though obviously based on the limited knowledge of the 13th century, was eventually borne out by modern scientific theories (it's possible to clone or "regrow" a body or any of its constituent parts from a piece of the original).
The deviancies of the Mutazilah actually arose more out of a faulty reasoning. That was compounded by the role they assigned to reason ('aql) over revelation. The Ash'aris had to basically not only put reason in its proper place, secondary to revelation, but also refute the faulty reasoning of the Mutazilah (their philosophy was wrong even without any theological considerations). This work is a part of the latter effort.
Philosophus Autodidactus ("Self-taught Philosopher")
Theologus Autodidactus ("Self-taught Theologian")
The differences between the sects/schools is apparent in the two works because at the end of Hayy ibn Yadqhan, Hayy realizes society basically sucks and takes a friend from the outside world back to the island where they live out the rest of their days in mystic contemplation. It suggests religious pluralism, as Mutazilah philosophy usually did, because they were unable to make effective arguments for Islam over other religions. It's no coincidence that other philosophers of that tradition (such as Ibn Rushd) actually kickstarted the "secular" tradition in Europe. Their works found an audience in Europe where those thinkers were already grappling with the glaring flaws in Christian theology. This was aided by the fact that Al-Andalus (Spain) was a hotbed of Mutazilah activity.
Philosophus Autodidactus had a profound impact on Europe. You can read about some of that at the Wikipedia page and also here.
Very few Europeans received later developments in the Muslim world because many of the Ash'ari and Maturidi theologians weren't in Spain but back in the Middle East. However, later developments did eventually reach Europe as well (Al-Ghazali's skepticism and Ash'arite doctrine of occasionalism did show up in Europe... occasionalism died out but the skepticism found a strong foothold in the works of George Berkeley and David Hume).
Such is the intellect of Ibn al-Nafis that some of his reasoning would be new even to today's Muslim "Rationalists" (of course it was, he was one of the major scientists of his era whereas today's rationalists' works are often worth less than the paper they are printed on). There is nothing fundamentally wrong with trying to scientifically analyze claims of religion (as the Ash'aris and Maturidis did, evidenced by this work) but the problem arises when one does not realize that this is error-prone subjective speculation and then changes the primacy of revelation over reason. When people do that, problems arise. For example, people who decide that Adam (as) had to have been evolved from other "near-humans" (that he had a mother and father). This is incorrect because the Scripture clearly states otherwise. But abiogenesis ("the study of how biological life arises from inorganic matter through natural processes") in and of itself isn't theologically incorrect at all. As for whether Adam was subject to it, we cannot say because we do not know if the "natural laws" or Allah's Customary Way (jary al-ada) of this world apply to Paradise, where Adam (as) was created but the process described in the hadith sounds very much like it.