《黄帝内经·素问·五常政大论》
黄帝道：在同一年中，有的动物能胎孕繁殖，有的却不能生育，这是什么气使它这样的呢？
帝曰：岁有胎孕不育，治之不全，何气使然？
The Yellow Emperor said: In the same year, some animals can become pregnant and breed, while others cannot bear young. What energy makes this happen?