"Human knowledge is of two kinds. One is knowledge of things perceptible to the senses-- that is to say, things which the eye, or ear, or smell, or taste, or touch can perceive, which are called objective or sensible... The other kind of human knowledge is intellectual-- that is to say, it is a reality of the intellect, it has no outward form and no place, and is not perceptible to the senses... (Even) nature... in its essence is an intellectual reality, and is not 'sensible'; the human spirit is an intellectual, not sensible reality. In explaining these intellectual realities, one is obliged to express them by sensible figures... Therefore to explain the reality of the spirit, its condition, its station, one is obliged to give explanations under the forms of sensible things. (6:1)

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