《公孙龙子·通变论》
偶蹄家畜所以不包括着奇蹄家畜，因为羊是种概念，牛是种概念，而偶蹄家畜却是类概念，因此如果把羊与牛概括为偶蹄家畜与奇蹄家畜划分开来，这是对的。
非马者，无马也。无马者，羊不二，牛不二，而羊牛二，是而羊而牛非马，可也。
The reason why double-toed domestic animals do not include odd-toed domestic animals is because sheep is a species concept, cattle is a species concept, and double-toed domestic animals is a class concept. Therefore, it is correct to summarize sheep and cattle as double-toed domestic animals, and odd-toed domestic animals, and distinguish them.