Yuan Zhen dared to speak of Emperor Xuanzong due to a combination of his personal courage, the political climate of the Tang Dynasty, and the specific circumstances surrounding his criticism. Yuan Zhen, a prominent Tang Dynasty poet and official, was not afraid to openly criticize the emperor, despite the potential consequences. This audacity can be attributed to several factors.

Firstly, Yuan Zhen's personal character played a significant role. He was known for his integrity and honesty, which allowed him to speak his mind without fear of repercussions. Yuan Zhen's willingness to challenge authority stemmed from his belief in moral and ethical principles, which he felt compelled to uphold even in the face of potential danger.

Secondly, the political climate of the Tang Dynasty provided a fertile ground for such criticism. The Tang Dynasty was a time of relative political stability and cultural prosperity, which allowed for a greater degree of intellectual freedom. During this period, scholars and officials were encouraged to express their opinions and engage in debates, fostering an environment where criticism of the emperor was not entirely unheard of.

Furthermore, the specific circumstances surrounding Yuan Zhen's criticism of Emperor Xuanzong were unique. Yuan Zhen's criticism was not a blanket attack on the emperor but rather a targeted critique of Xuanzong's policies and decisions. For instance, Yuan Zhen criticized the emperor's handling of the An Lushan Rebellion, which was a significant event during Xuanzong's reign. Yuan Zhen's criticism was based on his observations and analysis of the situation, which he felt compelled to share with the emperor.

Historical data supports the notion that Yuan Zhen's criticism was not without basis. According to the "Tangshu" (New History of the Tang), Yuan Zhen's poem "To the Emperor" directly addressed the emperor's shortcomings, particularly in relation to the An Lushan Rebellion. The poem criticized Xuanzong's decision to retreat to Sichuan during the rebellion, which was seen as a sign of weakness and indecision. This criticism was not only bold but also well-founded, as Xuanzong's retreat did indeed have negative consequences for the empire.

In conclusion, Yuan Zhen's daring to speak of Emperor Xuanzong can be attributed to his personal integrity, the political climate of the Tang Dynasty, and the specific circumstances surrounding his criticism. His willingness to challenge authority and share his observations with the emperor was a testament to his character and the intellectual freedom of the time. Historical data further supports the validity of his criticism, highlighting the significance of his actions in the context of the Tang Dynasty.