Hakuin Ekaku, also known as Hakuin Zenji, was a Japanese Zen master and reviver of the Rinzai school of Zen Buddhism in the 18th century. He is renowned for his teachings and writings that emphasized the importance of self-reflection, meditation, and direct experience of enlightenment.

One of Hakuin's most famous koans, or paradoxical riddles used in Zen practice, is the question, "What is the sound of one hand clapping?" This koan has been pondered and contemplated by Zen practitioners for centuries, and its meaning goes beyond the literal interpretation of the phrase.

At first glance, the question may seem nonsensical or impossible to answer. After all, how can one hand clap? However, Hakuin's intention was not to find a logical answer to the question, but rather to lead his students to a deeper understanding of the nature of reality and the self.

In Zen Buddhism, the concept of duality is a fundamental obstacle to enlightenment. This duality refers to the separation of the self and the external world, the subject and the object. The sound of two hands clapping represents this duality, the idea that there is a distinction between the clapping hands and the sound they produce.

By asking, "What is the sound of one hand clapping?", Hakuin is challenging his students to let go of this dualistic thinking and to experience the sound of one hand clapping directly. This experience goes beyond words and concepts and can only be understood through direct experience.

Hakuin's koan also highlights the importance of mindfulness and present-moment awareness in Zen practice. When one is fully present in the moment, there is no separation between the self and the external world. The sound of one hand clapping can only be heard in the present moment, not in the past or future.

Furthermore, the koan encourages introspection and self-inquiry. Hakuin is inviting his students to look within and question the nature of their own existence. What is the self that is hearing the sound of one hand clapping? Who or what is the one hand that is clapping?

In conclusion, the meaning behind Hakuin's famous saying, "What is the sound of one hand clapping?", is to lead his students towards a direct experience of the nature of reality and the self. It invites them to let go of dualistic thinking, be fully present in the moment, and engage in introspection and self-inquiry. It is a powerful reminder to seek enlightenment not through intellectual understanding, but through direct experience.