2 The Features of the Dao

Lao Zi's concept of the Dao serves as the keystone for his philosophy in general, and the starting point for his doctrine of the origin of the universe in particular. With high awareness of the duality of the Dao, known as Being-without-form (wu) and Being-within-form (you), Lao Zi exposes such general features as the Dao's imagelessness, soundlessness, formlessness, vagueness and elusiveness with regard to "the inseparable One" (i.e. the Dao) and their interactions with their counterparts. In this section we concentrate on chapters 14, 35, 21 and 5 (DDJ).
2.1 (Chapter 14)
You look at it but can not see it; It is called the imageless.[1]
You listen to it but can not hear it; It is called the soundless.[2]
You touch it but can not find it; It is called the formless.[3]
These three cannot be further inquired into For they are the inseparable One.[4]
The One is not bright when it is up, And not dark when it is down.[5]
Infinite and indistinct, it cannot be named, Thus reverting to a state of non-thingness.[6]
This is called shape without shape, Or image without image.
It is also called the Vague and the Elusive.[7]
When meeting it, you cannot see its head, When following it, you cannot see its back.[8] Hold on to the Dao of old,[9] In order to harness present things.
From this you may know the primeval beginning.
This is called the law of the Dao.[10]

In Tai chi, the beauty is this imageless/soundless/formless "object" you are dealing with during your taolu's performance. As [8] & [9} mention it, according to Tai Chi chuan, by seeking constantly this "object", you will develop further perception of your surroundings as a "door opener" of internal martial arts.

Annotations:
[1], [2] and [3] The three features of the Dao--"the imageless," "the soundless" and "the formless"--all reflect the subtlety of the Dao that goes beyond sensory perception.
[4] "The inseparable one" stands for the Dao itself. Similar terms are used in other contexts, for instance, in chapters 22 and 39 (DDJ).
[5] The Dao is not manifest or visible when it is without form; it becomes clear and perceivable when it is within form as a result of its transformation into De. These two aspects of the Dao, like the two sides of one coin, turn out to be identical to Being-without-form and Being-within-form, as discussed previously with regard to the nature of the Dao (see Part I, 1.1).
[6] The term "non-thingness" (wu) does not mean that there is nothing at all. Instead it denotes a state of being without shape. In other words, it refers to the existence of the Dao as the origin of all things, remaining unavailable to the senses.
[7] The Chinese concept hu huang is translated as "the Vague and Elusive," as two essential characteristics of the Dao. They themselves are compatible with the indescribable and unnameable features of the Dao (see Part I, 2.3).
[8] This illustrates the greatness of the Dao as it exists everywhere or embraces all things as a whole.
[9] "The Dao of old" (gu zhi dao) indicates that the existence of the Dao precedes those of all things in the world.
[10] Apart from this interpretation, rendered as "the law of the Dao," the original expression dao ji is also explained by some Lao Zi scholars as "the foundation of the Dao."

Commentary:
As depicted in this chapter, the general features of the Dao appear to be multi-dimensional. They can be generalized as imagelessness, soundlessness, formlessness, shapelessness, vagueness, elusiveness and namelessness. They are also described as invisibility, intangibility, indescribability and infinity. Yet, by scrutinizing them we may tentatively conclude that the Dao as such is characterized by these two fundamental aspects: firstly, the non-observable aspect as implied in "it is not bright when it is up;" that is to say, the Dao is invisible and indistinct when it is above form. It simply transcends the empirical and corporeal things as well as physical perception. Secondly, the observable aspect as suggested in "it is not dark when it is down." This means that the Dao becomes clear and manifest when it is within form, or, in other words, when it is transformed into De. These two aspects, non-observable and observable, could be likened somewhat to the metaphysical and physical concepts of occidental philosophy.
The greatness or infinite nature of the Dao is revealed in the passage, "When meeting it, you cannot see its head; when following it, you cannot see its back." This seems at the first sight to be

(1)(2)(3)(4)(5)