The True Nature of Mind Forms...
Remember, to receive and retain, to read and recite, and to think reasonably about [prajnā] are just to guard prajnā. And to want to guard it is to receive and retain it, to read and recite it, and so on.
~Shobogenzo, Maka-hannya-haramitsu, Gudo Nishijima & Mike Cross
To think about prajna (the wisdom of enlightenment) is to experience a mental image of prajna and to experience a mental image of prajna is to think about prajna.
Now, where is prajna apart from the mental experience of prajna?
Investigate this and you will personally verify the truth that the very substance of prajna is mind as it is.
Prajna does not exist independent of the mind as if waiting to be discovered. Pursuing this in practice brings realization that the true nature of all things, beings, forms, etc. (dharmas) is mind.
All dharmas (things, beings, etc.) are expressions of Buddha nature, expressions of the one mind, outside of which nothing exists. Dogen calls these expressions of mind, “the real form of all dharmas.” Thus Dogen describes, illumines, and clarifies the significance and implications of their true nature as mind – mental images, pictures, visions, expressions, paintings and the like.