埏埴以为器，当其无，有器之用。
We turn clay to make a vessel; but it is on the space where there is nothing that the usefulness of the vessel depends.
凿户牖以为室，当其无，有室之用。
We pierce doors and windows to make a house; and it is on these spaces where there is nothing that the usefulness of the house depends.
故有之以为利，无之用。
Therefore just as we take advantage of what is, we should recognize the usefulness of what is not.