《论衡·卷十六·讲瑞篇》
麒麟、凤凰这一类东西，前后出现的麒麟形体毛色各不相同，想以汉宣帝时所见到的高有五尺，羽毛有五彩花纹的凤凰作为标准，依据前面的标准去比较后面的，倘若再出现凤凰，就说它与宣帝时所见的凤凰相同，这就错了！
夫骐驎，凤皇之类，骐驎前後体色不同，而欲以宣帝之时所见凤皇高五尺，文章五色，准前况後，当复出凤皇，谓与之同，误矣！
For the same category of unicorns and phoenixes, the appearances and colors of the unicorns that appeared before and after were different. If we take the phoenix seen at the time of Emperor Xuan of Han Dynasty, which was five feet high and had colorful feathers, as the standard, and compare the later ones with the earlier ones, if a phoenix appears again, it would be wrong to say that it is the same as the phoenix seen at the time of Emperor Xuan.