Thirty spokes, forms a wheel, together with the empty space within, yields the functionality of a car.
-> Ancient vehicle were run on wooden wheel, with 30 spokes connected to a circle in the middle of the wheel. The forms (spokes) and non-form (empty space of the circle) together yields the functionality of the wheel/car.
Clay when fashioned into vessels, together with the empty space within, yields the functionality of a vessel.
-> Form (clay) and non-form (empty space within vessel) together yields the functionality of a vessel.
Doors and windows when cut to form a room, together with the empty space within, yields the functionality of a room.
-> Form (doors and windows and walls) and non-form (empty space within the room) together yields the functionality of a room.
Therefore, beingness and nothingness together offers functionality.
-> Beingness (form) and nothingness (non-form) cannot be separated in its functionality and manifestation in the world. Can a wheel function as a wheel without the empty space in its center? Can a vessel be used to contain water if there is no space within? Can a room serve its purpose if it is filled? Similarly, empty space itself cannot do the job (of a wheel, vessel, room) without their corresponding form counterparts.
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Chapter 11 Discussion:
Our mind likes to separate things into categories and opposites, as if the mentally separated subjects could exist independently. However, Laozi pointed out in this chapter, as a further elaboration to Chapter 2, with the 3 examples of wheel, vessel, and room, that it is a fallacy to separate our life into duality and opposites, and that separated and dualistic subjects do not have independent, stable and everlasting reality. Not only does the separation of form and non-form a fallacy to the perceptive world, it is also a fallacy to separate our own life into opposites. For example, in the classic duality of body vs. mind. Can the body be perceived without the mind? Can the mind exist without a body? The answer is obvious. Our life consists of both the body and the mind, which are two categories we created to describe certain aspects of our life (there is more, of course). What does it matter? It points us to perceive life as it is, beyond concepts and separation the mind created which does not stand at an objective and absolute level. It points us to go beyond duality and separation of the mind. It is the nature of higher truth, which when realized, liberates us from our ignorance, which is temporary, transitory, and illusive in nature. Therefore, in the Tai-Chi symbol, the black and white (or the yin and yang) are depicted separately yet are connected and united in the center to represent the highest truth of the world. Laozi did not teach Yin and Yang, he taught the truth beyond them.
Bilingual text of Chapter 11: http://www.lisiming.net/philosophy/chinese-philosophy/daoist/daoist-philosophy/dao-de-jing-core-33-chapters/ddj11-3/