Four

The Way is empty, but no matter how much you use it, it never runs out.
So deep, it seems to be the source of all things.

Blunting sharp edges,
Loosening knots,
Softening the glare,
Becoming one with the dust.

Like deep water the Way will endure forever.
I don't know whose child it is;
it seems to be older than God.


Commentary:
The idea that "The Way is empty, but no matter how much you use it it never runs out" is one that runs through the entire Tao Te Ching. What does it mean? Remember that the Way represents how the authors of the Tao Te Ching think the universe works. In fact, "the way the universe works" is one common definition of the character [Tao] used like this. If you understand the rules of the universe, you can use these rules forever and they won't run out.

Think of two automobile mechanics: One has a deep and intuitive understanding of how an engine works. When he fits two parts together, they fit perfectly. When he arranges moving parts, they have the right amount of clearance and slide over each other smoothly, without needing to be lubricated. The second mechanic doesn't have a good understanding of the engine, and his repairs are often adjusted poorly, with the moving parts of the engine rubbing on each other and generating friction and wear. He compensates for this by squirting a profuse amount of oil on everything he does, reducing the friction between parts. Although the first mechanic uses no lubricant, his engine is better calibrated because he understands, on a deep level, the rules and principles an engine runs by. This understanding, although he uses it every day, will never run out. The second mechanic's engine, however, needs to be constantly checked and re-lubricated because it is not adjusted according to the rules by which engines run. Replace the engine with the world, and you have an idea of what the authors of the Tao Te Ching probably have in mind when they talk about the Way being useful, but inexhaustible.