I Ching make pains to distinguish between conceptions of the Tao (sometimes referred to as "named Tao") and the Tao itself (the "unnamed Tao"), which cannot be expressed or understood in language. Liu Da asserts that the Tao is properly understood as an experiential and evolving concept and that there are not only cultural and religious differences in the interpretation of the Tao but personal differences that reflect the character of individual practitioners. The Tao can be roughly thought of as the flow of the Universe or as some essence or pattern behind the natural world that keeps the Universe balanced and ordered. It is related to the idea of qi, the essential energy of action and existence. The Tao is a non-dualistic principle—it is the greater whole from which all the individual elements of the Universe derive. Keller considers it similar to the negative theology of Western scholars, but the Tao is rarely an object of direct worship, being treated more like the Hindu concepts of karma, dharma, or Ṛta than as a divine object. The Tao is more commonly expressed in the relationship between wu (void or emptiness, in the sense of wuji) and yinyang (the natural, dynamic balance between opposites), leading to its central principle of wu wei (inaction or inexertion). The Tao is usually described in terms of elements of nature, and in particular as similar to water. Like water it is undifferentiated, endlessly self-replenishing, soft and quiet but immensely powerful, and impassively generous. Much of Taoist philosophy centers on the cyclical continuity of the natural world and its contrast to the linear, goal-oriented actions of human beings. In all its uses, the Tao is considered to have ineffable qualities that prevent it from being defined or expressed in words. It can, however, be known or experienced, and its principles (which can be discerned by observing Nature) can be followed or practiced. Much of East Asian philosophical writing focuses on the value of adhering to the principles of the Tao and the various consequences of failing to do so. The Tao was shared with Confucianism, Chán and Zen Buddhism, and more broadly throughout East Asian philosophy and religion in general. In Taoism, Chinese Buddhism, and Confucianism, the object of spiritual practice is to "become one with the Tao" (Tao Te Ching) or to harmonize one's will with Nature (cf. Stoicism) to achieve "effortless action" (wu wei).