As a scale-independent theory of deformation and fracture applicable to practical engineering problems, the field theoretical approach employed by physical mesomechanics is discussed in this paper. Being based on a fundamental physical principle known as gauge invariance, this approach does not rely on empirical concepts or phenomenology, and thereby is fundamentally scale independent. The derivation of physical-mesomechanical field equations is examined on a step-by-step basis, and the physical meaning of each step is clarified. Along the same line of argument, plastic deformation and transition to fracture is interpreted as an energy-dissipative process. Previously derived plastic deformation and fracture criteria are validated via detailed theoretical consideration and comparison with supporting experimental data.