Perfect Wisdom (from Zen Buddhism: A History, volume 1, by Prof. Heinrich Dumoulin S.J., translated by James W. Heisig and Paul Knitter, New York 1988):

In the Prajñaparamita sutras the significance of wisdom for the pursuit of salvation is evident. It is wisdom that sets the wheel of doctrine in motion. The new doctrine of the Wisdom school is thus considered by Mahayana to be the ‘second turning of the Dharma wheel’, second in importance only to the first teachings preached by Shakyamuni.

The Prajñaparamita sutras also set forth the evangel of the Buddha by claiming silence as their highest and most valid expression. Wisdom, all-knowing and all-penetrating, is deep, inconceivable and ineffable, transcending all concepts and words. Most important, wisdom sees through the ’emptiness’ (shunyata) of all things (dharma). Everything existing is always ’empty’. The broad horizon of meaning enveloping this word, which occurs throughout the sutras, suggests that, in the attempt to grasp its content, feeling must take precedence over definition. In the Heart Sutra, the shortest of the Prajñaparamita texts, wisdom is related to the five ‘skandhas’, the constitutive elements of human beings, and to all things contained in them. The sutra is recited daily in both Zen and other Mahayana temples, often repeated three times, seven times, or even more. In drawn out, resounding tones the endless chanting echoes through the semidark halls (…)

In the Wisdom sutras the stress is put on demonstrating the doctrine of the emptiness of ‘inherent nature’ (svabhava). Free of all inherent nature and lacking any quality or form, things are ‘as they are’ – they are ’empty’. Hence, emptiness is the same as ‘thusness’ (tathata), and because all things are empty, they are also the same. Whatever can be named with words is empty and equal. Sameness (samata) embraces all material and psychic things as part of the whole world of becoming that stands in opposition to undefinable Nirvana. In emptiness, Nirvana and Samsara are seen to be the same. The identity of emptiness, thusness, and sameness embraces the entire Dharma realm (dharmadhatu). Like the Dharma realm, Perfect Wisdom is unfathomable and indestructible. Here the doctrine on wisdom reaches its culmination.

Of special importance for Zen is the fact that Perfect Wisdom reveals the essence of enlightenment. As a synonym for emptiness and thusness, enlightenment is neither existence nor nonexistence; it cannot be described or explained. “Just the path is enlightenment; just enlightenment is the path” (Conze, Selected Sayings – see advayavada.org/excerpts.htm for more relevant excerpts).