Reason is the fourth source
Reason can be considered to be the fourth source of sciences and laws of religion. The most important distinction of man over animals is intellect or reason. Civilization and progress in natural sciences, technology, humanities, philosophy, Gnosticism, ethics, arts etc. are all contributions of the intellect of man. If reason and intellectual sources are taken away from man, he would lose all these perfections. Basically, the daily life of man is based on these same intellectual sources.
In Islamic culture also, intellect is introduced as a prominent and valuable entity, which can lead man in exposing realities. Islam is a religion of contemplation, rationalization and reason.
The Holy Quran in numerous verses has invited people to contemplation, pondering and reasoning and has condemned its absence:
إِنَّ فِى خَلْقِ الْسَّمَوَ تِ وَالْأَرْضِ وَاخْتِلَفِ اللَّيْلِ وَالنَّهَارِ وَ الْفُلْكِ الّتِى تَجْرِى فِى الْبَحْرِ بِمَا يَنْفَعُ الْنَّاسَ وَمَآ أَنْزَلَ اللَّهُ مِنَ الْسَّمآءِ مِنْ مَّآءٍ فَأَحْيَا بِهِ الْأَرْضَ بَعْدَ مَوْتِهَا وَبَثَّ فِيهَا مِنْ كُلِّ دَآبَّةٍ وَتَصْرِيفِ الْرِّيَحِ وَالْسَّحَابِ الْمُسَخَّرِ بَيْنَ الْسَّمَآءِ وَ الْأَرْضِ لَأَيَتٍ لِّقَوْمٍ يَعْقِلُونَ
“Most surely in the creation of the heavens and the earth and the alternation of the night and the day, and the ships that run in the sea with that which profits men, and the water that Allah sends down from the cloud, then gives life with it to the earth after its death and spreads in it all (kinds of) animals, and the changing of the winds and the clouds made subservient between the heaven and the earth, there are signs for a people who understand.” (2:164)
He also says:
وَ هُوَ الَّذِى أَنشَأَ لَكُمُ السَّمْعَ وَ الْأَبْصَرَ وَ الْأَفْئِدَةَ قَلِيلاً مَّا تَشْكُرُونَ * وَهُوَ الَّذِى ذَرَأَكُمْ فِى الْأَرْضِ وَإِلَيْهِ تُحْشَرُونَ * وَهُوَ الَّذِى يُحْيِىْ وَيُمِيتُ وَلَهُ اخْتِلَفُ الَّيْلَ وَالنَّهَارِ أَفَلَا تَعْقِلُونَ
“And He it is, Who made for you the ears and the eyes and the hearts; little is it that you give thanks. And He it is, Who multiplied you in the earth, and to Him you shall be gathered. And He it is, Who gives life and causes death, and (in) His (control) is the alternation of the night and the day; do you not then understand?” (23:78-80)
Numerous traditions have been recorded about the importance of intellect, reason and contemplation. The first topic of Al-Kafi is ‘The Book of Reason and Ignorance’. It is mentioned in it:
Abdullah bin Sinan has narrated from Imam Ja’far Sadiq (a) that he said: The Prophet is the proof of Allah on the people and reason is the proof between Allah and the people.
In the same way, he said: Reason is the guide of believers.
The Holy Quran, in addition to inviting the people to contemplation and pondering, has itself resorted to logical reasonings in many instances. The Prophet of Islam (s) and the Holy Imams (a) invited people to contemplate and proved their points through logical reasoning.
On the basis of this, the method of using reason and logical argumentation became acceptable in Islam and it urged people to follow this method. Therefore, reason became a reliable religious proof and a source of sciences and laws. It is possible to reach to the realities through intellect and logical reasoning. If intellectual sources are according to the criteria, they expose the reality and produce certainty. Although it is also not in the meaning that intellect in its own understanding, is basically free of error and its perceptions are always according to the situation, because existence of instances of mistake cannot be denied, but existence of limited instances of errors do not harm the root of the intellectual laws and do not dismiss intellect from the position of the finality of evidence. On the contrary, the cause of occurrence of these errors should be searched for in incorrect method of contemplation and reasoning. It is due to this that the science of logic has been developed.
Use of intellect
It is correct that intellect is a trustworthy and honest guide and it can be used to expose the realities, but it is not correct that we should wait for logical reasoning in proving every matter; on the contrary there are laws and judgments in special cases, which are briefly as follows:
Logical reasoning in fundamentals of belief
Religious beliefs can be divided into two groups: Fundamentals of beliefs (Roots of religion) and secondary beliefs (Branches of religion).
In Fundamentals of beliefs (Roots of religion), it is allowed to make use of logical reasoning; like in case of proving the existence of God; monotheism, positive and negative divine attributes, proving resurrection, necessity of sending of the prophets, proving the infallibility and knowledge of prophets, proving the origin of Imamate and necessity of appointment of the Imams and proving the knowledge and infallibility of the Imam.
In all the absolute fundamentals of belief, it is possible to make use of logical reasoning; that is why it is necessary to believe in those points of belief only after proper reasoning and contemplation. Islam in no way compels people to accept the fundamentals of belief without logical reasoning; on the contrary, it has urged them to ponder and comptemplate.
But in secondary beliefs, mostly there is no scope for logical reasoning, like the existence of purgatory, interrogation of the grave, process of establishment of Judgment Day, proving of deeds, accounting of deeds, balance of deeds, Sirat Bridge, true description of bounties of Paradise and chastisement of Hell etc. On the basis of this, it is not possible to understand these realities through intellect and logical reasoning alone; on the contrary, in these matters, it is necessary to rely on reliable religious texts. Same is the case with regard to the intercession of prophets, Imams and holy saints, existence of angels, existence of Shaitan and how he dominates human beings; the process of divine revelation; all these cannot be understood through logical reasoning alone, on the contrary, it is necessary to refer to religious traditional reports.
Logical reasoning in practical laws
It is also possible to use logical reasoning in deriving some practical obligatory laws. For example in the following instances:
First: Deriving the religious command in a subject from criterion and definite philosophy of a law in another subject; for example, there is an issue in scholastic theology believed in by most Muslims, especially Shia Imamiyah; and it is that laws of Shariah are framed keeping the real merits and demerits of a thing. That is if something is obligatory or prohibited in religious law, it is so because it is in fact necessary or harmful. In the same way, if something is recommended or detestable, it is because it is having an inherent merit or a demerit.
On the basis of this, if the legislator has framed law on a subject along with a cause, and intellect also creates the same cause in another topic, it necessitates that the second topic is also having the first command, even though it might not have reached us.
For example, the law maker says: Do not take wine, since it intoxicates. Now if intellect comes to know about the intoxicating qualities of other drinks, like Nabidh etc. it can declare that Nabidh is also prohibited. That is according to intellectual requirement, it says: ‘All that is ordered by intellect is also commanded by Shariah’. Therefore, it prohibits Nabidh also even though this command might not have reached us.
In the same way, if intellect finds a defect in an action, it orders against it. In that instance, according to requirement of intellect, it would be said that the legislator has also in this instance issued prohibition about it, because the legislator with regard to His servants, does not revoke any exigency. For example, if reason perceives that use of narcotics, like heroin is having individual and social ills, it orders against this act. In that instance, according to the rule of requirement of intellect, it would be said that addiction and use of heroin is also unlawful according the law of religion.
On the basis of this, in such instances, it is possible to conclude a religious command from logic; hence logic is a method of deriving a religious command.
Although the rule of requirement of intellect will be applicable in the instance and in the process of deriving the law of religion, it is determined that criterion and actual cause of the law, by the reliable legal reasoning should have been proved, or the existence of exigency or evil should have been proved from the definite law of intellect, but it is not possible to derive the law of Shariah through logical analogy and lenient interpretation.
Anyway, such derivation cannot be made by anyone except by a jurist, who is aware of the time, place and contemporary conditions.
Second: Conflict of two obligatory duties: For example, on one side the time of prayer is tight and only enough time is left to perform the obligatory part; on the other hand, a believer is drowning; in such a case, two obligatory duties have come upon one person: Both, performance of prayer and saving the life of a believer are obligatory; but he cannot fulfill both, and he is compelled to choose one. On this juncture, reason commands that saving the life of a believer is more important than prayer; thus he must give priority to it even if his prayer may lapse.
Third: Obligation of performing the prefaces of obligatory acts; that is if the act becomes obligatory, reason commands: It is obligatory for the obliged person to prepare the prefaces of that act, so that he may be able to perform the obligatory duty. For example, if Hajj is obligatory on a person, reason orders him to prepare the preliminaries of the journey and to travel to Mecca, so that he may wear the pilgrim garb at Miqaat and perform the obligatory rituals of Hajj. Completing the formalities is obligatory according to reason even if the Law Maker may not have specified it.
On the basis of this, reason is also a source of sciences and laws of Islam; with regard to fundamentals of belief, reason is having a decisive say and in deriving the laws also in its special instances, it becomes useful to the jurisprudents.
 Al-Kafi, Vol. 1, Pg. 25
 Al-Kafi, Vol. 1, Pg. 25
 Date wine
 Starting point of Hajj