An elegant and simple proof by Zhu Xi of the existence of Natural Principle (tianli 天理, the “Principle of Heaven”).
This Confucian doctrine of “reaching others from oneself” has been compared to the golden rule. Now, to “treat others the way you want to be treated,” you must assume some commonality, that people want the same things. Unless this assumption is true, the rule does not work.
The Confucian version reads: “To be able to draw a likeness from what is near at hand may be called the method of humanity” (Lunyu 6.28). As Zhu explains, “To draw a likeness of others from one’s own person is to liken what one desires oneself to other men, knowing that what they desire is just the same.” To then extend what one desires so as to reach others is the method of humanity (ren 仁).
Therefore, that this is even possible proves by inference that some common pattern, order, or principle flows through all men without interruption (and through all things, as man and nature form a unity). Li 理 (“principle”) is just a name for this observed natural standard. As it is formally defined, principle is the reason a thing is as it is, and the standard for what it ought to be.
Without this natural order, the whole business would not work.
To reach others from oneself is the heart and mind of the man of humanity. Looking at it from this point of view, one may see that the Principle of Heaven (tianli 天理) flows through all things without interruption. (Zhu Xi : Lunyu 6.28)
以己及人,仁者之心也。於此觀之,可以見天理之周流而無閒矣。