From Chapter 19 ("You Can't Get Off the Train") of No Beginning, No End: The Intimate Heart of Zen by Jakusho Kwong:

... What zazen really is has been explained in many different ways. Suzuki-roshi used to say shikan taza is our zazen, but as I've mentioned, the words and phrases used to express Zen practice can sound pretty abstract, and almost any explanation is conceptual. It can't really touch the experience itself. Suzuki-roshi put it very simply: "Just to be ourselves." Four words, but what they express is so deep and so subtle that we miss it until we begin slowing down. He said our practice is following our breath. But the true meaning of this, the deep meaning, is that you are vividly alive. The method says to follow your breath, but its meaning is that you are invisibly present and alive, just like a spinning top that goes faster and faster until it disappears. Without method, without expectation, without counting your breath, you are alive moment to moment; alive in this space that is nowhere else but right here within yourself. Others have said that zazen or shikan taza means being present in this very moment, but even that's not it. It's being the moment. It's being each moment after moment after moment in zazen, before zazen, and after zazen as well. And of course that's the manifestation or actualization of your original mind. "Just to be ourselves." This is shikan taza. ...

^z - 2017-05-05