Without a knowledge that there are levels, what they are and what they are like, what is to follow will be incomprehensible, since there are levels in everything that has been created; therefore they exist in every form. Consequently, I need to discuss levels in this part of Angelic Wisdom.
We can tell clearly from the angels of the three heavens that there are levels of love and wisdom. Angels of the third heaven so surpass angels of the second heaven in love and wisdom, and these in turn so surpass angels of the farthest heaven, that they cannot live in the same place. Their levels of love and wisdom mark them off and separate them. This is why angels of the lower heavens cannot climb up to angels of the higher heavens, and why if they are allowed to climb up they do not see anyone or anything around them. The reason they do not see anyone is that the love and wisdom of the higher angels is on a higher level, a level beyond their perception. Every angel actually is her or his love and wisdom; and love together with wisdom is human in form because God, who is love itself and wisdom itself, is human.
Occasionally I have been allowed to see angels of the farthest heaven go up to angels of the third heaven. When they managed to get there, I heard them complain that they could not see anyone; and yet they were surrounded by angels. They were afterwards told that these angels had been invisible to them because they could not perceive their love and wisdom, and it is love and wisdom that give angels their human appearance.
from Divine Love and Wisdom, Section 179