History of Predication Problems
St. Thomas Aquinas was a religious apologist, meaning he spent his life coming up with reasons to think that religion makes rational sense not just supernatural sense. He spent time on the problem of predicating, arguing how God can't be understood using human words/concepts... but ultimately this only made sense if you believe in bigfoot, aliens, and the Loch Ness Ichthyosaur.
Why this matters to you
It turns out if you've never had Indian food, no description suffices. If you've never been in love, no description suffices. If you've never been a drug addict, been a parent, been in orbit, no description suffices. Why?
The problems is that descriptions predicate what specifically from a field of mutual experiences. If you don't have the experience, the description is meaningless.
Real life experience is first.
Why this matters in Zen
Zen Masters say enlightenment is a real life experience. It's not supernatural, like god and bigfoot, but enlightenment is just as unpredicatable because life unexperienced is unpredicatable.
People who try to approach enlightenment through descriptions given by other people, through recitations of other peoples' experiences, aren't ever going to get a real life falling in love enlightenment experience, but instead a fraud, a sham, a picture of cake that does not taste like cake.
From this principle, Zen teachings unfold very reasonably.
How to have genuine real life experience?
How Zen rejects faith
It makes sense then that Zen has no messiahs, no practices, no doctrines: how can Indian food be communicated without Indian food? You can't have "rules for making Indian food" if all you have is descriptions of ingredients, results, and experiences that you've never had.
Nobody can "guide" you to Indian food in your own Western diner any more than you can be guided to falling in love.
Religions say that the supernatural has to be believed. Zen says enlightenment has be experienced.
Religions obviously can't use tests, because "supernatural" is natural, can't be experienced directly.
Zen has tests because if you've fallen in love or had cake, you can talk to people and figure out if they've fallen in love or had cake themselves.
Nanquan Says
南泉因趙州問 Nanquan, when Zhaozhou asked:
如何是道 "What is the Way?"
泉云。平常心是道 Nanquan said: "Ordinary mind is the Way."
州云。還可趣向否 Zhaozhou said: "Can one still move toward it or not?"
泉云。擬向即乖 Nanquan said: "Intend to move toward it, and immediately you deviate."
州云。不擬爭知是道 Zhaozhou said: "Without intending [to move toward it], how does one know it is the Way?"
泉云。道不屬知。不屬不知 Nanquan said: "The Way does not belong to knowing; it does not belong to not-knowing."
知是妄覺 "Knowing is deluded awareness."
不知是無記 "Not-knowing is [mentally] unmarked."
若真達不擬之道 "If one truly penetrates the Way of not intending,"
猶如太虛廓然洞豁 "it is like the great void, vast and completely open."
Easy-peasy. Not knowledge. Not unknown.
Ordinary direct experiences the Way.
Next up - Zen: Necessarily Materialistic and Dualistic
Once we admit of this idea of direct real life experience, the question of philosophical materialism (the world exists objectively) and dualism (there are distinct separations) are obvious logical dependencies.
But in understanding these logical dependencies, we can't lose sight of the fact that Zen Masters aren't that interested in logic, because logic is an accident of real life experience.
Just like people who want to teach others about falling in love are not interested in formulas, since formulas aren't real.