وَإِذَا قَرَأْتَ الْقُرْآنَ جَعَلْنَا بَيْنَكَ وَبَيْنَ الَّذِينَ لَا يُؤْمِنُونَ بِالْآخِرَةِ حِجَابًا مَّسْتُورًا 
The divine revelation for the human mind is like the external light to eyes. The light of the eye alone is not enough in the process of vision unless it is accompanied by an outer light.