The famous koan of the buffalo passing through the window is listed by Hakuin as a Nanto koan, or a koan that is particularly difficult to pass through. The difficulty though is not in the paradoxical nature of the image presented, but rather in coming to terms with what is realized in the koan. Most of us one way or another will think about practice in terms of a before and after - what I was like before, what I hope to be after. And when we have some kind of experience of passing though a barrier, of having the splits and confusions that brought us to practice seemingly dissolve for a little while, we quickly feel like we've gone from the before to the after. Yet in no time at all it seems like all the dualisms and splits reassert themselves. And so we get back on the cushion and practice harder. We want to do it again, but this time, once and for all. There is the beginning of the real difficulty Hakuin is pointing to.