  It is shown that the necessary criterion for choosing the energy-momentum
tensor of a physical system is the form of its linear invariant, which should
be the lagrangian of this physical system. Examples of energy-momentum tensors
that meet this criterion are given. For an electromagnetic field in vacuum, the
linear invariant of the energy-momentum tensor must be the canonical lagrangian
or the quadratic invariant of the electromagnetic field tensor. A mathematical
method for obtaining the lagrangian that complements the Lagrange formalism is
proposed.
