In connection with this idea of free development, Chuang Tzu makes a contrast between what is of nature and what is of man. "What is of nature," he says, "is internal. What is of man is external（天在内，人在外）...That oxen and horses should have four feet is what is of nature（牛马四足，是谓天）. That a halter should be put on a horse's head, or a string through an ox's nose, is what is of man.（络马首，穿牛鼻，是谓人）" (Ch. 17) Following what is of nature, he maintains, is the source of all happiness and goodness（顺乎天然，乃是一切快乐和善良之所由来）, while following what is of man is the source of all pain and evil（而服从于人为则是痛苦和邪恶的由来）.
(庄子秋水篇，河神河伯与海神北海若有一段对话，北海若说：“天在内，人在外，... 牛马四足，是谓天；络马首，穿牛鼻，是谓人。”）
Things are different in their nature and their natural ability is also not the same. What they share in common, however, is that they are all equally happy when they have a full and free exercise of their natural ability(万物本性和天赋能力各有不同。共同点是：当它们充分并自由发挥天赋才能时，便同样感到快乐）. In "The Happy Excursion" a story is told of a very large and a small bird. The abilities of the two are entriely different. The one can fly thousands of miles, while the other can hardly reach from one tree to the next. Yet they are both happy when they each do what they are able and like to do. Thus there is not absolute uniformity in the natures of things, nor is there any need for such uniformity. Another chapter of the Chuang-Tzu tells us: "The duck's legs are short, but if we try to lengthen them, the duck will feel pain. The crane's legs are long, but if we try to shorten them, the crane will feel grief. Therefore we are not to amputate what is by nature long, nor to lengthen what is by nature short."(ch 8) （庄子骈拇篇：凫脛（脚骨）虽短，续之则忧；鹤胫虽长，断之则悲。故性长非所断，性短非所续，无所去忧也。人要想戕贼万物本性，强求一致，是徒劳无益的。）
