If seeing the cow's face in the picture at the left is analogous to kensho, it is certainly not dualistic to say there is, at first, seemingly, no cow in the picture—then a cow appears. What changes is not the given picture, itself, but how we perceive that which is in front of us. In the example of kensho, that is, seeing our true nature which is Buddha Mind, before we saw this Mind we did not see it. Now we see it everywhere. What is more, there is no real duality between Mind as suchness and phenomena, anymore than there is a duality between water (HOH) and waves. It is our deluded mind that is the problem which will not open the door to truth.
Dogen Zenji seems to have had a problem with kensho which he saw as being dualistic. In Francis H. Cook's paper (1983), Enlightenment in Dogen's Zen (JIABS vol. 6 no. 1) we learn of Dogen's idea of Buddhism.
Dogen suspected the Platform Sutra of being a forgery, and part of the reason for this is that its teaching of kensho (見性) seemed to be at variance with what Dogen saw as the true situation. The idea of kensho, "seeing one's nature," implies a very fundamental dualism, in that there is a "nature" which is Buddha, and something else that "sees." Consequently, there is a fundamental dualism of Buddha and not-Buddha. However, Dogen's point d'appui for everything he had to say about the Buddha Way was the understanding that there is only Buddha, and therefore the assertion of something or someone seeing Buddha contradicted his understanding.
What Cook is saying can be seen here. Dogen asserts that "All sentient beings without exception are Buddha-nature" (Shobogenzo, Bussho). However, to get to this assertion Dogen, purposely, distorts a critical passage from the Mahayana Mahaparinirvana Sutra which reads, "All sentient beings without exception have the Buddha-nature." (Italics are mine.) The "have" denotes potentiality whereas the "are" means everything is Buddha-nature. For Dogen there is no true nature (性) for us to see (見)—it's all right here when, for example, we look at a donkey's jowls! With Dogen's rejection of kensho, by implication, he is also rejecting Zen which is based on kensho: "Seeing our true nature becoming buddha" (見性成佛).
Dogen seems not to want to accept that among other things, purification is necessary. With a purified mind we experience kensho. We see that this nature has never not been present. Our non-seeing of it owes to our own ignorance and confusion. In chapter 8 of the Lotus Sutra, 'The Concealed Gem', we learn that a priceless gem was sewn into the lining of a poor man's clothes by a rich man. Regrettably, the poor man was drunk at the time and had no idea that in his clothes there was a priceless gem. Then the rich man had to leave the poor man. Meanwhile, the poor man unaware of this treasure continues his life of poverty until one day he again meets the rich man who shows him the gem concealed in his clothing.
In respect to this parable, the poor man is rich and sentient beings are Buddhas but neither of them know this. For Dogen this is good enough—no need to go any further. Dogen is suggesting that the unenlightened mind is the Buddha Mind.