The mechanics of solids theory from the 18th C is obsolete since energetic physical thinking, thermodynamics, and bonds in solids were discovered in 1840-1870. Elastic deformation is by nature a change of state in the sense of the First Law of thermodynamics; this cannot be concluded from the common theory. A proper approach to elastic deformation – stress – must begin with the equation of state, and be based on the First Law. – The thermodynamic theory (in scalars P, V, T) is isotropic by nature. It has been transformed into vector field form (f, r, T) to consider anisotropic boundary conditions. The displacement field has a contracting eigendirection [ED] at 111°, an extending ED at 11° to the foliation plane. EDs can neither rotate nor shear. The maximum shear directions, the bisectors between the EDs, are also the directions of maximum angular velocity. Therefore, the common idea of crystallographic glide planes aligning with the bulk shear direction, thereby permitting unlimited dislocation glide, is unrealistic. Not the glide planes control the fabric orientation, but the maximum material anisotropy aligns with the extending ED. The crystals populating a maximum are in a locked position, they cannot glide. It follows that extended plastic flow by dislocation glide alone is physically impossible. Secondary processes must be active simultaneously, such as grain boundary glide, recovery, heterogeneous shear zones. The new approach correctly predicts all observed features of plastic simple shear, such as S‑C, sigma-delta clast systematics, and microfabric obliquity.