Divine things and religious truths are only known from the Prophet (peace be upon him). He knows them best and is most competent to expound them.
Things of the world which are the objects, for example, of medicine, mathematics and commerce are known through reason.
Knowledge is what is demonstrable, and of that what is useful has been conveyed by the Prophets (peace be upon them).
However, there is a part of knowledge which we get from other sources. This concerns the matters of the world, such as the objects of medicine, mathematics, agriculture and commerce.
But so far as divine things and religious truths are concerned, the only source of their knowledge is the Prophet (peace be upon him). He knows them best, he is most eager to preach them to the masses and most competent to formulate and expound them. He is above everyone in knowledge, will and competence, things that are required to accomplish his mission perfectly.
Everyone else is deficient in knowledge, has a distorted idea of things, or lacks the urge to preach what he knows either because he is seeking something else, entertains some fears or does not have sufficient power to expound his ideas clearly and forcefully to the conviction of the people.
Sometimes the Prophet advances rational arguments in support of the truths he preaches. The Quran is full of rational arguments and clear proofs regarding divine realities and religious truths.
Sometimes the Prophet only states them, for he has already offered clear and convincing proofs to establish the fact that he is a prophet of God bearing a message from Him, that he is telling nothing about God except the truth, and that God has Himself borne witness for him and told people that he is honest and reliable in delivering His message to them.
In fact, there are many arguments to establish that he is the Messenger of God; some of them are rational and verifiable by reason, others are religious and revealed, but the Prophet explained them to people and demonstrated them.
Theologians of different affiliations are agreed that the Quran offers rational arguments on religious matters. They often mention them in their theological and exegetical works. They also make use of religious and revealed arguments in various matters; since they have established the veraciousness of the Prophet, he must be believed, they say, in whatever he states.
Knowledge is of three kinds. One is not known except through rational arguments. Concerning this category, the best arguments are those which the Quran has stated and the Prophet has mentioned.
Let it be known that the best and the most perfect rational arguments are the ones which are received from the Prophet. I emphasize this fact because many people do not know it. Some people reject outright all rational arguments, since they believe that they have been manufactured by the theologians. Others do not reflect on the Quran, or try to understand the rational and convincing arguments which it offers, because they have somehow developed the idea that the Quran only states truths.
They think that one should first ascertain the veracity of a Prophet and the authenticity of the words he has said on rational grounds, and then deduce the truth of his statements from the truth of his prophethood.
The other kind of knowledge is that which a non-Prophet has no way of knowing except through a Prophet; his word alone is the argument for it. This knowledge is concerned with details regarding God, angels, the Throne, Paradise and Hell, as well as details regarding things which the Prophet enjoins or forbids.
As for as the existence of the Creator and His unity, knowledge, power, will, wisdom, and mercy, these matters can be known through reason. But the arguments and the proofs which the Prophet offers in these matters are the most perfect and rational arguments.
However, the prophetic word is not the only means for knowing them, even though it does offer certain knowledge. These things are, therefore, known by the rational arguments which the Prophet offers as well as by his word, since his veracity has been established by arguments, proofs and miracles.
People have also differed concerning knowledge of the Hereafter and knowledge of good and evil. The majority is of the view that they are known through reason as well as revelation (as- sam`).
However, those who uphold the rationality of the knowledge of good and evil are more than those who uphold the rationality of the knowledge of the Hereafter.
Abu Al-Khattab noted that this is the view of most of the jurists and theologians. The other view is that we know about the Hereafter and about good and evil from no other means than the word of the Prophet. This is the view of Al-Ash`ari and his followers, as well as many others from the followers of the scholars, such as Qadi Abu Ya`la, Abu Al-Ma`all Al-Juwayni, Abu Al-Walid Al-Bajl, and so on.
However, both groups agree that there are things that are known through reason as well as through the word of the Prophet, such as the issue of whether human acts are created by God or not, whether we shall see Him in the Hereafter.
What I want to say is that one should take from the Prophet the knowledge of all divine and religious matters, both those that are merely revelational and those that are rational, and base upon it all one’s rational arguments, since what he has said is true in principle as well as in detail.
Arguments for the veracity of the Prophet establish the truth of his teachings in principle, and the rational arguments that the Quran and the hadith expound demonstrate that in detail.
Again, Prophets and messengers of God are sent first of all to give this very knowledge; they are, therefore, the most knowledgeable on these matters, the most competent to teach them, and the most honest about them.
Those who examine what they say and what the others say in such matters find that the truth is with the Prophets and that others are mistaken. This is asserted by Ar-Razi, who is extremely critical of arguments from tradition and is known to have made the statement that arguments from tradition fail to produce conviction. No other recognized scholar has come out with such a scathing remark.
But it is he who is reported to have said, “I have long pondered theological and philosophical arguments, and have finally reached the conclusion that they do not generate certainty. In comparison to them, the arguments of the Quran are more convincing. Read, for example, the verse, “To Him mount up (all) words of honor.” (35:10), or the verse, “The Most Gracious One is firmly established on the Throne” (20:5), which describe God in positive terms. Read also the verse, “There is nothing whatsoever like unto Him” (42:11), which describes Him in negative terms.”
Thereafter, he said, “Whoever goes through the experience I have gone through will reach the same conclusion I have reached.”
Similarly, if you consider those who do not follow the Prophets nor have faith in their teachings, you will find them skeptical, perplexed and ignorant of the truth, or ignorant as well as conceited. They are like those about whom the Quran has said,
But those who disbelieved – their deeds are like a mirage in a lowland which a thirsty one thinks is water until, when he comes to it, he finds it is nothing but finds Allah before Him, and He will pay him in full his due; and Allah is swift in account. Or [they are] like darknesses within an unfathomable sea which is covered by waves, upon which are waves, over which are clouds – darknesses, some of them upon others. When one puts out his hand [therein], he can hardly see it. And he to whom Allah has not granted light – for him there is no light. (An-Nur 24:39-40).
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Source: Taken with modifications from a book entitled “Ibn Taymiyyah expounds on Islam” which is a translation of some selected fatwas of Shaykh Al-Islam Ibn Taymiyyah, translated by Muhammad `Abdul-Haqq Al-Ansari. The fatwa is numbered 13:136-141 from Al-Fatawa by Ibn Taymiyyah.