There's a great line in The Zen Teaching Of Huang Po (as translated by John Blofeld) that goes like so:
If you WILL conceive of a Buddha, YOU WILL BE OBSTRUCTED BY THAT BUDDHA!!! And when you conceive of sentient beings, you will be obstructed by those beings.
CAPS EMPHASIS!!! are Blofeld's. (His translation is not always the most accurate, but for readability and spirit it's still great.)
This is one of those lines that people trip over when they are relatively new to Buddhism and Zen specifically, but it's really not that complicated. All he's saying is, the ideas that we have in our minds about Buddha, or Zen, or enlightenment, or ourselves, or anydamnthing at all, are not the same things as the things themselves. We use our ideas about things and our perceptions of them to get a little closer to an understanding of those things, but our ideas and our understandings must always be regarded as tentative and provisional.
Ideas about Buddhas and sentient beings get singled out for special treatment in the above quote, I think because they are a special source of confusion for would-be Zen practitioners. Throughout the book there are countless passages that insist on the idea that Buddha, the enlightened state, is not some special, far-away, exotic realm of mind — it's something that exists in, undergirds, and overlays every conscious moment we have. It's always there, always a part of us. It's just that we're not in the habit of paying attention to it or letting it come to the fore, and we live in a world that offers us plenty of alternatives (read: distractions) from ever having to do that.
Our ideas about things are a big part of how we got here as a species. They're also a big part of what keeps us from making progress as a species, because we tend to give the ideas a life of their own they don't really deserve. "It's absurd to be loyal to a style," Milton Glaser once wrote, in regard to visual fashion. "It does not deserve your loyalty." An idea deserves your attention and your consideration, but your devotion ... well, that's another story. It seems unwise to love something that is incapable of loving you back, for one.
"All such dualistic concepts as ‘ignorant' and ‘Enlightened', ‘pure' and ‘impure', are obstructions," is the sentence that immediately follows the quote above. " ... Just as apes spend their time throwing things away and picking them up again unceasingly, so it is with you and your learning. All you need is to give up your ‘learning', your ‘ignorant' and ‘Enlightened', ‘pure' and ‘impure', ‘great' and ‘little', your ‘attachment' and ‘activity'. Such things are mere conveniences, mere ornaments within the One Mind. I hear you have studied the Sūtras of the twelve divisions of the Three Vehicles. They are all mere empirical concepts. Really you must give them up!"
Attacks on our ideas about these things are constant throughout the work — not the things themselves, but the way our ideas about them end up hijacking the conversation, the way thinking about them and talking about them become weak substitutes for the actual experiences of those things. But it also, I think, constitutes instructions to the Zen practitioner to not think about their zazen work in a qualitative sense. Don't sit there and try to compare today's zazen with yesterday's zazen (which is why I find ideas like "levels of samadhi" to be actively harmful; they gamify something that shouldn't be gamified). Don't let the scholarship become more important than the actual doing of the thing, which is how the scholarship began anyway. The best way to know about enlightenment is to sit down and let your own embodiment of it come through, something you don't need to explain to anyone, certainly not yourself.
Most of us know the pop-culture bastardization of the above: "If you meet the Buddha on the road, kill him." I hated how that was often used to reduce Zen to the level of a punchline, and a smarmy nihilistic one at that (two things Zen for sure aren't). The truth inside that silly phrase remains, though. If you meet the Buddha on the road, look inside instead.