It concerned -- or named -- his theory that a poet doesn't need or desire to nail everything down, nor to understand anything through and through. That is, I take it, a poet acknowledges the impossibility of so doing, and writes within that limitation

The term you are looking for is "intuetics", inselpeter.** Nowadays it is referred to as "obscurantism" ... without any credit to Yeats. Defn. "A style in art and literature characterized by deliberate vagueness or obliqueness." [A-H]

And St. Paul said it best, long before Yeats, methinks. [Nostradamus and the priestesses of the Delphic Oracle had a leg up on Yeats, as well.]

to see through a glass darkly
To see �through a glass��a mirror��darkly� is to have an obscure or imperfect vision of reality. The expression comes from the writings of the Apostle Paul*; he explains that we do not now see clearly, but at the end of time, we will do so.

http://www.bartleby.com/59/1/throughaglas.html

* 1 Corinthians 13:12
For now we see through a glass, darkly; but then face to face: now I know in part; but then shall I know even as also I am known.

**
"I still, however, tend to see the tower as a symbol of �reason,� reason as opposed to �intuition.� The rational thinking of scientists and scholars is continually opposed to the inspiration or intuition of the artist in Yeats� poetry, as it was in almost all forms of romanticism. The tower is a symbol I will need to re-examine and come back to later. Maybe next time around I�ll argue that the tower symbolizes Yeats� view that art should be practiced as an act of life, not removed from the world."

http://lorenwebster.net/In_a_Dark_Time/archives/000111.html