A new proof of the positive energy theorem in classical general relativity is presented. The theorem states that the total energy of a gravitating system is always positive, except for Minkowski space, which has zero energy. This result has been previously established by Schoen and Yau using a different method. The paper also presents a new proof that there are no asymptotically Euclidean gravitational instantons. These results are relevant to the stability of Minkowski space as the ground state. In classical field theories, the total energy is usually the integral of a positive definite energy density. However, in gravity, the energy density of the gravitational field cannot be defined in a satisfactory way. The total energy of a gravitating system can be defined in terms of the asymptotic behavior of the gravitational field at large distances. It is an old conjecture that this total energy is always strictly positive, except for Minkowski space. Various approaches have been used to study the positive energy problem. These include the study of gravitational waves, the analysis of special classes of gravitating systems, and the use of variational arguments. Schoen and Yau used a geometrical method to prove the positive energy theorem for the key case of a space with a maximal spacelike slice. They generalized their proof to a general proof of the positive energy theorem, thus finally resolving this long-standing problem. The paper also discusses the stability of Minkowski space against semiclassical decay processes. It is shown that such a decay does not occur in pure gravity. The new proof of the positive energy theorem shows that Minkowski space is the unique space of lowest energy in classical general relativity, providing a more far-reaching indication of its stability than the absence of a semiclassical decay mechanism.A new proof of the positive energy theorem in classical general relativity is presented. The theorem states that the total energy of a gravitating system is always positive, except for Minkowski space, which has zero energy. This result has been previously established by Schoen and Yau using a different method. The paper also presents a new proof that there are no asymptotically Euclidean gravitational instantons. These results are relevant to the stability of Minkowski space as the ground state. In classical field theories, the total energy is usually the integral of a positive definite energy density. However, in gravity, the energy density of the gravitational field cannot be defined in a satisfactory way. The total energy of a gravitating system can be defined in terms of the asymptotic behavior of the gravitational field at large distances. It is an old conjecture that this total energy is always strictly positive, except for Minkowski space. Various approaches have been used to study the positive energy problem. These include the study of gravitational waves, the analysis of special classes of gravitating systems, and the use of variational arguments. Schoen and Yau used a geometrical method to prove the positive energy theorem for the key case of a space with a maximal spacelike slice. They generalized their proof to a general proof of the positive energy theorem, thus finally resolving this long-standing problem. The paper also discusses the stability of Minkowski space against semiclassical decay processes. It is shown that such a decay does not occur in pure gravity. The new proof of the positive energy theorem shows that Minkowski space is the unique space of lowest energy in classical general relativity, providing a more far-reaching indication of its stability than the absence of a semiclassical decay mechanism.