Know that for the human mind there are certain objects of perception which are within the scope of its nature and capacity; on the other hand, there are, amongst things which actually exist, certain objects which the mind can in no way and by no means grasp: the gates of perception are closed against it. Further, there are things of which the mind understands one part, but remains ignorant of the other; and when man is able to comprehend certain things, it does not follow that he must be able to comprehend everything.