Most tanka contain two poetic images. The first is taken from nature; the second, which may proceed, follow, or be woven into the first, is a kind of meditative complement to the nature image. Tanka produce a certain dreamlike effect, presenting images of reality without that definite quality of "realness" often possessed by photographs or drawings, as if the images proceeded directly from the mind of the dreamer. The tanka poet may be likened to a person holding two mirrors in his hand, one reflecting a scene from nature, the other reflecting himself as he holds the first mirror. The tanka thus provides a look at nature, but it regards the observer of nature as well.
I found this description within Richard von Sturmer's judge's report of the 3rd Kokako Tanka Competition, in Kokako 14 (April, 2011) and had to share it. It's the philosophy of tanka writing that I was taught in a workshop with Owen Bullock in December 2009. I've been writing tanka ever since.