(4) In terms of the Dao as the essence of all things, it is the unity between the vital force (qi) as the world's physical base and the natural law of its changes.
(5) The Dao is the imperishable, essential in the physical world, and therefore all things arc subordinate to the law of the Dao, which is powerful enough to clear all barriers from its path.
(6) The Dao functions in this manner: All things and phenomena are in constant motion and change, in the process of which they are transformed into their own opposites.
(7) All things and phenomena are involved in a kind of interrelationship which is accomplished via the unifying Dao.
(8) The Dao is neither visible nor tangible; it is beyond our sensory perception but yet cognitive by means of logical thinking. (see Yang Xingshun. Zhongguo Gudai Zhexuejia Lao Zi Jiqi Xueshuo [Ancient Chinese Philosopher Lao Zi and His Doctrines])
Following are more interpretations of the Dao offered for reference:
The word Dao is, according to Prof. Fung Yu-lan, one of the most important terms in Chinese philosophy. It has a primary meaning of "road" or "way." From this primary meaning it assumed already in ancient times a metaphorical significance, as the "way of man," that is, human morality, conduct or truth. During this time, its meaning was always restricted to human affairs, whereas when we come to the Dao De Jing, we find the word Dao being given a metaphysical meaning. That is to say, the assumption is made that for the universe to have come into being there must exist an all-embracing first principle, which is called Dao (see Fung Yu-lan. A History of Chinese Philosophy. p. 177).
The Dao, as a philosophical concept initially put forth by Lao Zi, carries two basic meanings: it sometimes indicates the substance of the physical world, that is, the noumenon of the universe, but in most cases it means the universal law that governs the motion and change of nature or reality. These two aspects tend to be so much entangled in Lao Zi's notion that the character of the law gets confused with its manifestation (see Zhang Songru. Lao Zi Jiaodu [A Revised Reading of Lao Zi's Dao De Jing]).
According to Tong Shuye, the idea of the Dao in The Book of Lao Zi is developed from the notion of Ming (Fate) in the pantheism prevailing since the Spring and Autumn Period (770-476 B.C.) in Chinese history. Ming was the negation of the concept of Tian (Heaven) and Gui (Ghost) available in the religious idealism of the past. As a result of denying the existence of God with will and personality and other deities, ghosts and spirits, there was no longer any master of the universe. Hence the ancient thinkers had to look beyond Tian and Gui for another universal master to govern the world in general and all human changes in particular. Thus emerged the theory of Ming as reflected in the pantheism. The notion of the Dao was thus based on the further abstraction of the theory of Ming, which can be seen as a natural product of the developmental process of thinking experienced by the ancient Chinese thinkers (see Tong Shuye. Xianqin Lao Zi Sixiang Yanjiu) A Study of Lao Zi's Thought in Pre-Qin Dynasty Times]). The Dao is also considered as the highest category of Lao Zi's philosophy. Reading the Dao De Jing, we find, according to Prof. Ren Jiyu, that the Dao has five distinct meanings: (1) It implies "the undifferentiated primitive state (chaos);" (2) It indicates "the motion of nature;" (3) It is "the proto-material;" (4) It is "invisible to man's eyes and imperceptible to the other sense organs;" and (5) It means "the law of all things." However, Lao Zi's Dao is "merely a preliminary supposition about the proto-material that forms all things, and Lao Zi himself had not yet the capacity to understand matter in general. Therefore, he puts forward the concept of undifferentiated (chaos) in his philosophical conception. The undifferentiated cannot be named: It is called `the nameless' or `simplicity'” (see A Taoist Classic: The Book of Lao Zi, pp. 4-5).
By means of substantial research into Lao Zi's way of thought and taking account of many other individual findings by Chinese scholars, Prof. Chen Guying has arrived at the conclusion that the Dao is characterized by several denotations to be apprehended accordingly in specific contexts. He consequently classifies the Dao as a metaphysically reality existent in some cases, as a type of universal law of things in other places, and lastly as an underlying rule or standard of human conduct and personal cultivation under certain circumstances. The Dao as presented in Chapter I implies a metaphysical character due to its being indescribable, unnameable and imperceivable by the senses, and having no definite form, even though it has a real existence and dynamically serves as the beginning of the universe (see Chen Guying. "Lao Zi Zhexue Xitong De Xingcheng" [The Development of Lao Zi's Philosophical System], in Lao-Zhuang Xinlun [New Essays on Lao Zi and Zhuang Zi]).

[2] In the original text the expression chang dao (constant Dao) is changed to be heng dao (eternal Dao) on the basis of the two copies of The Book of Lao Zi written on silk and unearthed in 1973 from an ancient tomb at Mawangdui, which dates back to the early Han Dynasty (c. 206 B.C.-180 B.C.). The tomb is located near Changsha, capital of Hunan Province. The relics found there contain two largely similar versions of the Dao De Jing which are considered by Chinese scholars as the earliest text of the book found so far. It is thought that the character heng was altered to chang simply to avoid the political taboo of repeating the name of Liu Heng, Emperor Wen of Han at the time the traditionally used text was written. However, both chang and heng mean the same in Chinese and they can therefore be translated into English as either "constant" or "eternal."
"Constant Dao" suggests such features of the Dao as eternity, indescribability, profundity, subtlety, irreplaceability and imperishability, etc. This is because the Dao, a universality of all changes, remains constant or eternal for ever, present in the myriad things as its manifestations move, change and generate along with the Heavenly way, natural law, or time and space.

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