The Sanlun school, also known as the Three Treatise school, is a Chinese Madhyamaka school that emerged during the Tang Dynasty in China. This school is based on the teachings of the Indian philosopher Nagarjuna and his interpretation of the concept of emptiness. One of the key concepts in the Sanlun school is the "threefold truth," which refers to the understanding of reality as consisting of three levels of truth.

The first level of truth in the Sanlun school is called the conventional truth. This level of truth refers to the world as we perceive it through our senses. It is the everyday reality that we experience and understand through concepts and language. According to the Sanlun school, this level of truth is not completely false, but it is also not completely true. It is a relative truth that is dependent on our perception and understanding.

The second level of truth is called the ultimate truth. This level of truth refers to the true nature of reality, which is emptiness. Emptiness, in this context, does not mean nothingness or non-existence, but rather the absence of inherent existence. The Sanlun school teaches that all phenomena are empty of inherent existence, and this emptiness is the ultimate truth. This understanding of emptiness is based on the concept of dependent origination, which states that all phenomena arise and exist in dependence on other causes and conditions.

The third level of truth in the Sanlun school is called the middle truth. This level of truth is the reconciliation of the conventional truth and the ultimate truth. It is the understanding that both levels of truth are necessary and interdependent. The middle truth recognizes that while the conventional truth is not completely true, it is still a valid way of understanding the world. At the same time, the ultimate truth of emptiness cannot be fully understood without the conventional truth.

The concept of the threefold truth in the Sanlun school is important because it helps practitioners avoid the extremes of nihilism and eternalism. Nihilism is the belief that nothing exists, while eternalism is the belief in an eternal, unchanging reality. The threefold truth teaches that reality is neither inherently existent nor non-existent, but rather a combination of both conventional and ultimate truth.

In conclusion, the concept of the threefold truth in the Sanlun school emphasizes the importance of understanding reality at different levels. It teaches that the conventional truth, ultimate truth, and middle truth are all necessary for a complete understanding of reality. By embracing these three levels of truth, practitioners can gain a deeper understanding of the nature of reality and ultimately achieve liberation from suffering.