‘Xuansha was informally addressing his monastics when he heard a swallow singing. He said to the assembly, “This is the profound dharma of real form. It skillfully conveys the essence of the true teaching.” He then descended down from the teaching seat.
A monastic asking for an explanation said, “I don’t understand.”
Xuansha said, “Go away. No-one will believe you.”‘ (Shinji Shobogenzo)
The commentary on this koan by Daido Loori points to Xuansha’s ‘This is’ as not being limited to the swallow or to Xuansha.
Meanwhile, the monk gets stuck in his head. As Dogen says so often, we should study this closely. Don’t try to think your way through any of this.