It is at this point that he comes upon a winged figure standing with one foot in a brook, the other on a rock. The radiant creature pours something from one flask into another. Drawing closer, the Fool sees that what is being poured from one flask is fire, while water flows from the other. The two are being blended together!
"How can you mix fire and water?" the Fool finally whispers. Never pausing the Angel answers, "You must have the right vessels and the right proportions." The Fool watches with wonder. "Can this be done with all opposites?" he asks. "Indeed," the Angel replies, "Any oppositions, fire and water, man and woman, thesis and anti-thesis, can be made to harmonize. It is only a lack of will and a disbelief in the possibility of unity that keeps opposites, opposite." And that is when the Fool begins to understand that he is the one who is keeping his universe in twain, holding life/death, material world and spiritual world separate. In him, the two could merge, as in the vessels that the Angel uses to pour the elements, one to the other. All it takes, the Fool realizes, is the right proportions....and the right vessel.
Belief that fiery red and watery blue cannot be merged may be the only thing standing in the way of blending the two.
This is one of the hardest cards to interpret. I think, perhaps, Crowley is most helpful in understanding it, as he calls the card: "alchemy." It sometimes works best for me to imagine the Angel wearing a lab coat and very carefully pouring measured amounts of colored liquids into beakers rather than cups.