Tao te Ching: 14

We look at it and do not see it;
Its name is The Invisible.
We listen to it and do not hear it;
Its name is The Inaudible.
We touch it and do not find it;
Its name is The Subtle (formless).
These three cannot be further inquired into,
And hence merge into one.
Going up high, it is not bright, and coming down low, it is not dark.
Infinite and boundless, it cannot be given any name;
It reverts to nothingness.
This is called shape without shape,
Form (hsiang) without object.
It is The Vague and Elusive.
Meet it and you will not see its head.
Follow it and you will not see its back.
Hold on to the Tao of old in order to master the things of the present.
From this one may know the primeval beginning [of the universe].
This is called the bond of Tao.

Finally we come back to subtlety, something we first touched upon in chapter 1. The Tao is something that we cannot see, hear, or touch—but it is still there. We simply cannot know more about it. This is because it is Nameless, and if we start naming, measuring and quantifying, we create all the little things under Heaven and Earth.

If you elevate the Tao, it isn't any brighter and if you use it every day, it isn't any the worse for wear. Many chapters have touched upon the idea that the Tao is a tool that can be used without depletion. Here we see that it isn't any more useful or visible if we elevate it.

Hsiang is interesting. Here it means form, but it is deeper than that. Hsiang is a physical manifestation of nature. Hsiang is used in it's traditional sense here, so that it means feature, appearance or form. So this form without object is the ultimate subtlety because we have many ways to describe it, but there isn't any "thing" there to describe. Taoists found this to be much more important than any manifestations of the Tao.

How different this is from Confucianism! In Confucianism, manifestations are key. They said there is nothing more manifest than the hidden (which we take to mean subtle here), and in The Mean, they say that a man who knows the subtle can enter into virtue.

The Neo-Confucianists took another turn and said there was no difference. In The Complete Works of the Two Ch'engs, Ch'eng I says "Substance and function come from the same source, and there is no gap between the manifest and the hidden." This is taken from the I chuan's preface. The Buddhists said much the same, but I haven't found the actual text on that. It may be in a book that I didn't bring to lunch.