In "Republic," Plato outlines a conception of human intelligibility that would define European philosophical thought for more than 1,000 years. For Plato, humans were able to comprehend the world around them due to a form of innate knowledge called forms. Objects in physical reality strive to recreate a perfect conception that exists just beyond the physical realm. For example, a sphere in the physical world is an imperfect copy of the ideal form of a sphere. That is not to say that forms are created by the human mind or even a divine being. They have a direct reality, in some ways more real than the physical worlds. They are living ideas inherent in all things and all thoughts to which corporeal reality struggles to conform. In this way humans understand the world through reference to these innate forms.
Building on the work of Plato, Mulla Sadra, the great Muslim scholar of the Golden Era, incorporated a conception of the divine into a theory of human intelligibility. To Mullah Sadra, existence is the primary reality, preceding and surviving essence, or the soul. As the creator of existence God is supreme and immutable, but it also allows essence an independence of will as a kind of secondary creation. The human soul tries to understand reality, reality responds to this action by making itself intelligible. In the act of trying to understand the human soul in turn makes itself intelligible to God, creating a grand unity of reciprocal intelligibility.
Following his famous epiphany in 1629 that "I think therefore I am," Rene Descartes took the view that thought is the first and primary evidence of reality. The evidence of the senses, however, does not necessarily provide an intelligible view of reality. Instead, humans must rely on intellectual perception, our ideas about what constitutes reality rather than just sights and sounds that can be deceiving.
Immanuel Kant suggested that human intelligibility essentially derives from a priori knowledge, i.e. an independent, divinely supplied conception of reality that is informed but not constituted from experience. In other words, God provides the intellectual mechanism by which we can translate our experience of the world into an intelligible understanding. Examples of a priori knowledge might be our ability to conceive of time, morality or cause and effect.