Knowledge can be classified into two- tangible or apparent knowledge and spiritual knowledge. The first one is that which can be acquired by anyone. Scientific knowledge comes under this category. It wouldn't be wrong if it is named sensation or perceptible knowledge. Experience is its basis. Referring tangible or apparent knowledge to sensation or perceptible knowledge doesn't chain it up in the five sensational knowledge. In addition to the knowledge provided by the senses and physical organs, the unintrospective knowledge that the mind provides also comes within its boundary. The theories that are shaped in the minds of the scientists while trying to assemble factual knowledge is, also, a part of tangible or apparent knowledge. Even though theories are evolved out of intuitions formed while scientists beat their brain over the questionnaire, the basis of these intuitions is knowledge and studies. Due to this, there is no essence in taking these intuitions beyond tangible knowledge. Verily, Human potential is limited. Our brain makes out things with the knowledge gained using the five sense organs. These senses, too, are limited. Only those lights with wavelengths between 380 and 780 milli microns can be perceived by the human eye. The rays with wavelengths greater or lesser than this range, like the ultra violet rays, X rays, infra red rays, cosmic rays are not perceivable for the human eye. Human ear hears only those sounds within 20 and 20,000 Hz. Ultra sound waves are insensible to our ears. Similar is the case of human taste as well as smell.
Since our senses are limited, the brain that synthesizes the information handed over by these senses to make decisions, should also be limited in some manner. In what way is the brain limited? It can only think and manipulate something that has substance. Our brain can neither comprehend nor create hypothetical picture of anything that is without substance. In other words, it can deal only in a time space continuum. Brain is not designed to deal with anything beyond time space continuum. Spiritual knowledge, the second kind, is not something that can be attained through self effort. According to the Islamic concepts, it is the prophets chosen by Allah who spread spiritual knowledge to the world. The prophethood itself cannot be attained as and when anyone wants. It is a Godly gift. Allah selects the ones whom he desires to entrust him with His message. More rightly put forward, He moulds into a prophet, the ones whom He has selected. In the holy Quran, Allah says ‘I sent' while referring to the prophets. This makes clear that prophet hood is not a status that can be attained out of self effort.  “We sent not a messenger, but to be obeyed, in accordance with the will of Allah.”(Qur'an 4:64) The prophets were assigned to teach the reality beyond tangible or apparent knowledge. Their duty was to spread knowledge that the human cannot attain through experience. Since they were enriched with this knowledge through divine revelation (wahi), they were completely assured of its authenticity. To make the people informed, aayahs (divine proofs) were revealed to the prophets. These aayahs served as tangible proofs that the prophets were receiving revelation. The only path open in front of man is to study and realise the divine source behind these aayahs and accept what the prophets point out as spiritual knowledge. It is one among the basic believes of Islam that prophet Muhammad was the last one to be gifted with divine revelation (wahi). He was the last prophet assigned to deliver spiritual knowledge unto the last man on earth. With him ended the long series of prophets that Allah assigned to lead the humans. The Quran that Allah revealed and the life of the prophet are the two resources of spiritual knowledge. For a Muslim, no other source can be accepted authentically.
We witness many people looking upon personal experience as the prime source of spiritual knowledge. However, these experiences cannot be proved experimentally. One's experience cannot be tested by someone else to prove its authenticity. Anyone can easily lie about his own experience and there is no means to spot it out. Moreover, even if they are sincere in quoting their observation, there is no way to distinguish whether they are divine plots or devils' to lead us astray. Lot of such difficulties will arise when the source of spiritual knowledge fails to be experimented. As a result, Islam stands against considering mere personal experiences as a source of spiritual knowledge. When Islam rejects personal experiences being sources of spiritual knowledge on the ground that they are not provable, the sources that Islam points out, the Quran and the Sunnah, should be and is indeed testable. On applying upon Quran, any test apt to prove a source to be divine, Quran will splendidly emerge with flying colours. The case of Sunnah is no different. It can be proved without a trace of doubt that prophet Muhammad (saw) was the messenger of Allah. The words and the mode of life that is reported to be of the prophet can be scientifically tested to prove that those are his. The Holy Quran instructs mankind to test these two sources and once realised, to accept these as scriptures of spiritual knowledge. Islam refers to the sole entity who created and sustains everything as Allah. The fact that He is unrivalled in his acts comes within the circle of apparent knowledge. But how can we know the character of His entity? It requires only common sense to realise that the creator of the world of substance will transcendent substance. It is therefore impossible to experience this entity with our senses.
The tangible knowledge, that which man can acquire, is that which is perceived by the senses. Man stands different from and superior to animals due to his gifted brain that allows him to interpret and conclude results based his observations via his senses. Yet we lag in the efficiency of the senses when compared to them. Dog's sense of smell, Eagle's eye sight and bat's sense of hearing stands above ours. However none of the animals have the ability to process the observed information to use and deliver it in a better way. Here lies the superiority of man...the excellence of his brain. He can develop and interpret beyond the information that his senses catch. Yet, no one can deny the fact that our brain, too, is limited. The raw materials for the brain are contributed by the senses. Since senses deal only with the material world, it is indeed impossible for it to process Him who is beyond a material. Here is where the brain restrains itself. This says without further explanation that man is incapable to provide self explanations and conclusions regarding Allah's entity. This underlines the fact that Islam considers nothing other than Qur'an and Sunnah as sources revealing knowledge about the creator Allah. Muslims realise these sources to be revealed from the Almighty Lord and thus accept it whole heartedly. The knowledge revealed by the Lord through the Quran and through the words and lifestyle of His messenger has to be accepted as such without enlisting extra meanings and definitions that turn up in human brain. All ways diverging from this are based purely on assumptions and hence prone to mistakes.