戰國時荀子曾說過：「殺人者死，傷人者刑」，對照漢摩拉比法典中所謂的「以牙還牙，以眼還眼」，這種「極致對稱」的觀念，不由得令人想起了腦神經中的鏡相神經元（mirror neuron）的作用。根據維基百科全書對鏡相神經員的定義：
A mirror neuron is a neuron which fires both when an animal performs an action and when the animal observes the same action performed by another (especially conspecific) animal. Thus, the neuron "mirrors" the behavior of another animal, as though the observer were itself performing the action. These neurons have been directly observed in primates, and are believed to exist in humans and in some birds. In humans, brain activity consistent with mirror neurons has been found in the premotor cortex and the inferior parietal cortex. Some scientists consider mirror neurons one of the most important findings of neuroscience in the last decade.
在放諸四海皆準的人性或風俗背後，竟能找出生理上的基礎，的確讓人感到踏實以及「學而時習之」（Once you can apply what you've learned, how great you'll feel）的暢快。
