If I traveled back to the Tang Dynasty, all I really wanted to be was a carefree prince.

The night he returned from Da'an Palace, Li Ke sat in a side hall for a long time.

The candlelight flickered, casting dappled shadows on the wall. A sheet of paper lay before him, detailing the dangers of consanguineous marriage—cousins ​​were too closely related, their children were more susceptible to congenital diseases, and the mothers were more prone to premature aging. He understood these words, but his understanding alone was insufficient. He needed to make his father and mother understand as well, to let them know that marrying Li Lizhi to Changsun Chong was not finding her a good match, but rather harming her.

But how could he convince his father? He couldn't simply say, "I read some books when I was studying medicine." His father wouldn't believe him. He needed evidence, concrete, tangible cases.

Li Ke stood up, walked to the window, and gazed at the Taiye Pond outside. Moonlight shimmered on the water, but his heart was unsettled. He recalled Li Chengqian's words at the Da'an Palace earlier that day—"Third brother, whatever you're going to do, count me in."

Yes, he also has an older brother.

Li Ke turned and left the side hall, striding towards the East Palace. Li An followed behind, jogging along the way.

In the Eastern Palace, Li Chengqian was still awake. He sat at his desk, several scrolls laid out before him, but he couldn't concentrate on reading a single word. Earlier that day at Da'an Palace, Li Lizhi had cried, saying she didn't want to marry into the Changsun family, and he felt bad about it too.

"Big brother." Li Ke's voice came from outside.

Li Chengqian looked up and saw Li Ke walk in, his expression grave. "Third brother, what's the matter so late?"

Li Ke sat down opposite him, closed the door, and lowered his voice, saying, "Brother, I've been thinking about my younger sister's matter all night. I can't handle it alone. I need your help."

Li Chengqian looked at him, his gaze becoming serious. "You speak."

"Brother, my younger sister doesn't want to marry into the Changsun family, not just because she doesn't like Changsun Chong." Li Ke looked into Li Chengqian's eyes. "Cousins ​​marrying each other means they're too closely related by blood. Children born from consanguineous marriages are prone to strange illnesses—some are born weak, some don't grow up, and some have intellectual disabilities. If my younger sister marries into that family, it won't just be her who suffers, but also her children."

Li Chengqian frowned. "Are you telling the truth?"

"When I was studying medicine, I read some books in the Imperial Medical Academy. The books explained it very clearly." Li Ke paused, "But books alone are not enough. Father Emperor won't believe the books of a child like me. I need evidence."

"What evidence?"

"Cases of consanguineous marriage. The Tang Dynasty is so vast, it's impossible that there weren't cases of cousins ​​marrying and having children. In some cases, the children were weak and sickly, and in others, the children were born with problems. As long as we find these cases and present them to the Emperor, he won't be able to disbelieve them."

Li Chengqian was silent for a moment, then nodded. "What do you want me to do?"

"Brother, you are the Crown Prince, and the Eastern Palace has extensive connections. Your people can investigate much faster than I can do it myself. I only need you to do one thing for me—send people to the clinics and pharmacies in Chang'an and the surrounding areas to find experienced doctors and ask them if they have seen any cases of cousins ​​getting married and having children born weak and sickly. The more cases, the better, and the more detailed the better."

Li Chengqian stood up, paced back and forth in the room a few times, then stopped. "Third brother, you're right. I'll handle this. I'll give you results within three days."

Li Ke stood up and bowed to Li Chengqian. "Thank you, elder brother."

"What are you thanking me for?" Li Chengqian patted his shoulder. "She's my sister too."

Li Chengqian kept his word. Early the next morning, he sent several trusted confidants from the Eastern Palace to inquire at various clinics and pharmacies throughout Chang'an. These men were all veterans of the Eastern Palace, tight-lipped, efficient, and unlikely to leak any information.

Meanwhile, Li Ke wasn't idle either. He went to the Imperial Medical Academy and borrowed several medical books from Wang Yongzheng. Wang Yongzheng asked him what books he wanted, and he said he wanted to look up some records about pregnancy-related illnesses. Wang Yongzheng didn't ask any further questions and took a few old volumes from the bookshelf and handed them to him.

Li Ke returned to the side hall and began flipping through the pages. Although ancient medical books lacked the concept of modern genetics, they contained numerous records of "fetal weakness" and "fetal illness." He found several cases—children born to parents who were close relatives were congenitally weak, sickly, and died young. He copied down these cases, recording them in detail, intending to present them to his father, the Emperor, later.

On the evening of the third day, Li Chengqian sent someone to summon Li Ke.

When Li Ke arrived at the Eastern Palace, Li Chengqian had a thick stack of papers in front of him, covered with dense writing. His expression was somewhat grave.

"Third brother, look at these."

Li Ke sat down and turned the pages one by one. The first page read: "A doctor at a pharmacy in the eastern market of Chang'an, with thirty years of medical experience, has seen three cases of cousins ​​marrying and having children. In the first case, the child was born weak and died at the age of three. In the second case, the child was born deaf and mute. In the third case, the child has intellectual disabilities and is still unable to take care of himself."

On the second page, an old doctor at a clinic in the West Market said that when he was young, he practiced medicine in Luoyang and saw a family of cousins ​​get married and have three children, none of whom lived past the age of five.

On the third page, a midwife in the south of the city said that she had delivered the babies of a pair of cousins, who were stillborn and their bodies were purple.

Li Ke turned the pages one by one, his heart growing heavier with each turn. Not because the cases were shocking, but because he knew they were just the tip of the iceberg. Many more cases remained unrecorded.

"Brother, that's enough." Li Ke closed the stack of papers. "This is enough to convince Father. But just convincing Father isn't enough—he might think this is just a matter for an ordinary family and has nothing to do with the royal family. We need to let Father know that this matter concerns not just our younger sister, but the entire Tang Dynasty."

Li Chengqian looked at him: "What do you plan to say?"

"Please, Father, issue a law prohibiting marriage between people within five degrees of kinship. The reason is not for my younger sister, but for the continuation of the Tang Dynasty and the stability of the nation. This way, Father will not think we are pleading for my younger sister, but rather for the sake of the country. The Changsun family will also have nothing to say—the law is set by the court, not targeted at anyone."

Li Chengqian's eyes lit up: "That's a great idea. Let's go see Father together first thing tomorrow morning."

The next morning, Li Ke and Li Chengqian went to the Imperial Study.

After Zhang De announced their arrival, the two men went inside. Li Shimin was reviewing memorials when he saw his two sons arrive together. He put down his pen and leaned back in his chair.

"Chengqian, Ke'er, what brings you two here together?"

Li Ke glanced at Li Chengqian, who nodded slightly. Li Ke stepped forward and knelt down.

"Father, I have something to report. I have thought about this for a long time, as it concerns the well-being of all people and the continuation of the Li family's bloodline. I dare not remain silent."

Li Shimin looked at him, his gaze turning serious. "What is it? Stand up and speak."

Li Ke did not get up. He took out the stack of papers from his sleeve and presented them with both hands.

"Father, this is what my elder brother and I have found out these past few days. Please take a look."

Zhang De took the stack of papers and placed them on the dragon's desk. Li Shimin picked them up and flipped through them page by page. His brows furrowed more and more, and by the time he reached the last page, his face had darkened.

"What is this?" Li Shimin put down the paper and looked at Li Ke. "Why are you investigating these things?"

Li Ke raised his head and looked into Li Shimin's eyes.

"Father, when I was studying medicine at the Imperial Medical Academy, I read some medical books. They said that children born to people who are too closely related by blood are prone to strange illnesses—some are born weak, some don't grow up, and some have intellectual disabilities. I didn't believe it at the time, so I asked my elder brother to help me investigate. As a result, we found that there were many such cases in Chang'an. Children born to cousins ​​who married either died young or were weak and sickly. Father, these doctors and midwives, they can't read or write, but what they said was all based on their own eyes."

Li Shimin remained silent for a while, looking at the stack of papers without saying a word.

Li Ke continued, "Father, I am not trying to criticize anyone. I simply feel that this matter concerns the well-being of all people. If the people of the Tang Dynasty engage in incestuous marriage, and the children born are inferior to one generation after another, how can the Tang Dynasty remain stable? Father, I humbly request that you issue a law prohibiting marriage between people within five degrees of kinship. This is not for any particular person, but for the continuation of the Tang people and for the stability of the nation."

Li Shimin tapped his fingers lightly on the table. His gaze shifted from Li Ke to Li Chengqian, then back to the stack of papers.

"Chengqian, did you have these things investigated?"

"Yes." Li Chengqian knelt down. "Father, after my third brother told me about this, I also felt that it should not be taken lightly. I sent people to investigate for three days, and this is what they found. Father, I dare not say that every case is related to consanguineous marriage, but with so many cases before us, I feel... we cannot disbelieve it. I beg Father to issue a law to rectify the world."

Li Shimin picked up the stack of papers again and flipped through them. He recalled some things—there had been children in the palace who were born weak and sickly, and there had been children who never grew up. He hadn't thought much of it before, but now, after reading these cases, he felt a vague unease.

"Ke'er, you said that if people who are too closely related by blood marry, they will have children with problems. How closely related by blood is considered too close?"

Li Ke knew what his father was asking, and he felt a little nervous, but he didn't show it on his face.

"Father, I read in a medical book that there's no problem once the five degrees of mourning are crossed. Within the five degrees of mourning, the blood ties are too close. Beyond the five degrees of mourning, the blood ties are more distant, so there's less risk. I beg you, Father, to issue a law prohibiting marriage between people within the five degrees of mourning. This is not only for the sake of all the people, but also for the sake of the Li family's bloodline."

Li Shimin nodded without asking any further questions. He put down the stack of papers and looked at his two sons.

"You two, have you been busy with this these past few days?"

Li Ke and Li Chengqian exchanged a glance, then both lowered their heads and remained silent.

Li Shimin sighed. "Get up. Don't kneel anymore."

The two stood up.

"I have taken note of what you have said. I will discuss the matter of promulgating the law with my court officials. Go back and study hard, and do not mention this matter again."

"Yes," the two replied in unison, then turned and walked out of the imperial study.

After leaving the imperial study, Li Chengqian let out a long breath.

"Third brother, do you think Father will issue a new law?"

Li Ke thought for a moment and said, "Father won't issue the decree immediately, but he will discuss it with the court officials. Once this precedent is set, my younger sister's marriage won't be settled so quickly. The Changsun family won't dare to bring it up again—who would want to bear the stigma of 'incestuous marriage, harmful to offspring'?"

Li Chengqian nodded.

The two walked side by side along the edge of Taiye Pond. An autumn breeze blew across the water, carrying a hint of coolness. The wind ruffled the water, reflecting the hazy sky, and ripples spread out in layers.

"Third brother," Li Chengqian suddenly said, "do you think Father and Mother will still betroth our eldest sister to Changsun Chong?"

Li Ke remained silent for a moment. "Brother, I don't know. But at least Father now knows the dangers of consanguineous marriage. Now that he knows, he'll think more about it. And once the law is enacted, the Changsun family will have nothing to say."

Li Chengqian stopped and looked at Li Ke. "Third brother, our efforts these past few days haven't been in vain."

Li Ke smiled.