Human's knowledge of the external world is acquired through our senses and is essentially relative and uncertain. This may lead us to doubt the existence of everything in the material world as a reality. Instead, in a skeptical view, everything is fictitious or mirage.
Rene Descartes, the French philosopher ,who is "credited as a foundational thinker in the development of Western notions of reason and science"according to the page below, developed and answered this argument by resorting to his most basic instincts in his famous line "I think, therefore I am." http://newlearningonline.com/new-learning/chapter-7/descartes-i-think-therefore-i-am
I believe Descartes' proposition primarily and indirectly proves that our most fundamental source of knowledge is ,in fact, our conscience which is synonymous with our existence. That is the knowledge that comes instinctively and naturally with no efforts. I also believe that "conscience" is commonly referred as"heart" and it is precisely equivalent to "Nafs" in Quran.
Here is the definition of the word "conscience" from the Longman Dictionary of Contemporary English: "the part of your mind that tells you whether what you are doing is morally right or wrong."
And these are two of the verses about "Nafs" in Quran which clarifies the fact that it is indeed God who inspires our Nafs (Conscience) resulting in a clear or in a troubled conscience.
بِسْمِ اللَّـهِ الرَّحْمَـٰنِ الرَّحِيمِ
"And [by] the soul and He who proportioned it." 91:7"And inspired it [with discernment of] its wickedness and its righteousness." 91:8
Speaking of inspiration, the major breakthroughs in human's knowledge has always rooted in an inspiration according to the history of science. Although it is true that usually inspirations occur after hard work and long hours of concentration on a specific problem, they come when we feel we are stock in a dead end situation. Needless to say, all forms of art are also undeniably associated with the inspiration that the artists feel in their heart.
Furthermore, all the rules of logic ,which is the basis for the entire human's knowledge from philosophy to mathematics, from natural sciences to the humanities, are based on the most basic question of " Does it make sense or not? " which we answer intuitively without any thinking.
In other words, the foundational principals of our knowledge whether it is based on data collected from our senses or it is the result of a thought experiment merely in our mind, are all based on the basic rules that are hardwired in our existence. It is only the findings of this core in us that brings us certainty and peace as clearly seen in the case of Descartes's quest for the Truth.
Perhaps we can then, rightfully, conclude that our heart / conscience is indeed the primary source of knowledge regardless of being conscious of it or not.
Lastly, we need to differentiate the findings of our heart from the product of our mind by the fact that the former one comes naturally, effortlessly and intuitively while the latter is the result of a thought process and some analysis. Finally, we may be able to distinguish between the two by noticing that what we find in our heart is the only absolute perception from the whole world internal or external to our-self.
