不知最親切 says that not knowing can bring you closer than anything else. The not-knowing it means is the kind you reach by letting everything go. A blank mind that simply does not know has missed it, and the koan's own commentary says so plainly. 親切 here keeps its older sense, close and right on the point, not the modern kind. Whether it is for your practice or for a season when you cannot see the way is yours.
The line is the master 桂琛's reply to the young monk 法眼文益, who, asked on his pilgrimage what the pilgrimage was about, had said he did not know; the exchange was first written up, with 法眼 waking at the line, in the 景德傳燈錄 of 1004, about fifty years after 法眼's death in 958.
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