Sosan, also known as Sengcan, was a Chinese Zen teacher who lived during the 6th century. He is considered the Third Zen Patriarch because he played a significant role in the development and spread of Zen Buddhism in China.

Sosan's early life is shrouded in mystery, and there are conflicting accounts of his background and upbringing. Some sources claim that he was born into a wealthy family and received a traditional Confucian education, while others suggest that he was a poor peasant who later became a monk. However, what is known for sure is that Sosan was deeply influenced by the teachings of the Second Zen Patriarch, Huike.

After meeting Huike, Sosan became his disciple and studied under him for many years. Huike recognized Sosan's potential and imparted upon him the teachings of Zen, particularly the concept of "sudden enlightenment." This concept emphasizes the idea that enlightenment can be achieved in a single moment of realization, rather than through gradual practice and study.

Sosan's most significant contribution to Zen Buddhism was his writing of the "Xin Xin Ming," or "Verses on the Faith-Mind." This text is considered one of the most important Zen texts and is still studied by practitioners today. In this text, Sosan emphasizes the importance of having a clear and open mind, free from attachments and dualistic thinking. He also stresses the idea of non-duality, the belief that all things are interconnected and part of a larger whole.

Sosan's teachings and writings had a profound impact on the development of Zen Buddhism in China. He attracted many disciples and followers, who spread his teachings throughout the country. His emphasis on sudden enlightenment and the importance of self-reflection and introspection greatly influenced the direction of Zen practice in China.

Today, Sosan is revered as one of the most influential Zen teachers and is considered the Third Zen Patriarch in the lineage of Zen Buddhism. His teachings continue to inspire and guide practitioners in their pursuit of enlightenment and inner peace.

In conclusion, Sosan was a Chinese Zen teacher who played a crucial role in the development and spread of Zen Buddhism in China. His teachings, particularly the concept of sudden enlightenment, continue to influence practitioners today. He is considered the Third Zen Patriarch due to his significant contributions to the Zen tradition and his impact on its development.