  We define a new gauge independent quasi-local mass and energy, and show its
relation to the Brown-York Hamilton-Jacobi analysis. A quasi-local proof of the
positivity, based on spacetime harmonic functions, is given for admissible
closed spacelike 2-surfaces which enclose an initial data set satisfying the
dominant energy condition. Like the Wang-Yau mass, the new definition relies on
isometric embeddings into Minkowski space, although our notion of admissibility
is different from that of Wang-Yau. Rigidity is also established, in that
vanishing energy implies that the 2-surface arises from an embedding into
Minkowski space, and conversely the mass vanishes for any such surface.
Furthermore, we show convergence to the ADM mass at spatial infinity, and
provide the equation associated with optimal isometric embedding.
