There are no artificial barriers or gates in Zen Buddhism that one has to pass through. Yet, as the Mumonkan informs us, there is a barrier (關). This is Joshu's "Mu," or for that matter any concept that looms before our mind's eye (one time Joshu said, "Yes," to the same question as to whether or not a dog has the Buddha-nature).
Even though true reality is gateless (無門), a universe of concepts occludes the gateless true reality which is pure Mind. These concepts of ours are looking for something to grab onto. We are of the persuasion that the awakened Mind, is conceptual. It can be roped like some calf in a rodeo show given enough rope and enough time. But it never works out this way. Our wanting to conceptualize the gateless true reality is a testament to man’s stubbornness and stupidity.
The remedy, as Zen sees it, for penetrating through the barrier of concepts that occlude the supreme vision, is not to meet that concept-barrier with still another such barrier, but to focus in on Mu, directly, because like every conditioned thing in the universe, there is a moment when it hasn’t yet arisen—it is unborn. It is the non-conceptual, unstirred Buddha Mind. This is called the hua-t’ou—let’s call it the ante-Mu. There is no barrier here. Not yet; not until we conceptualize another Mu again (and again, and again). At some point we have to grow tired of conceptualizing; of holding onto the barrier Mu or any of the koan phrases which we believe we can answer conceptually.
Our temporal world, including our conceptual thoughts, is not continuous. Our thoughts arise and perish. The problem for us is we do not know from whence they arise and into what they disappear. If we did, there would be no barrier. We would instantly see that the Mu barrier which stands in front of the gateless is a composition of the true gateless reality which is pure Mind. When we conceptualize, Mind is hidden from us—the still water is stirred, so to speak. By awakening to pure Mind we discover the very substance of our conceptual thoughts. Mu is just a mode of this substance.