I believe that it is crucially important for us as zazen practitioners to distinguish zazen as the entirety of Buddha Dharma from shuzen as one genre of it, even though these two practices look similar at a glance. We should avoid confusing them. That is why Dogen repeatedly emphasized this point (zazen is not shuzen) in his writings (Fukanzazengi, Shobogenzo, Eihei Koroku, etc..). It could be said that the bulk of his wirings were written to clarify the criteria for discerning authentic zazen.
Then, what is the difference between zazen and shuzen? This is a very important question to consider when we practice zazen. Even if we are sitting with almost the same posture, it does not mean the content is also the same (“If there is a hairsbreadth deviation, it is like the gap between heaven and earth” Fukanzazengi). I am wondering how many zazen practitioners are keenly aware of the importance of this question.
This is easy to say but very difficult to do for us because we are usually driven by a desire to achieve something which does not exist here and now. When we hear that zazen is about no achievement, we immediately ask, “If zazen is that, how can I do it?” But this is a question exactly stemming from the framework based on “means and end” which is always behind the shuzen approach. It is nothing but an undertaking to grasp zazen using the shuzen concept. This shuzen attitude is deeply rooted in our way of behavior and thinking. That is why we should take a radically different approach to zazen so that we can avoid changing zazen into shuzen, consciously or unconsciously.