The nature of elastic deformation is examined in the light of the potential theory. The concepts and mathematical treatment of elasticity and the choice o f equilibrium conditions are adopted from the mechanics o f discrete bodies, e. g., celestial mechanics; they are not applicable to a change o f state. By nature, elastic deformation is energetically a Poisson problem since the buildup of an elastic potential implies a change of the energetic state in the sense of thermodynamics. In the Euler-Cauchy theory, elasticity is treated as a Laplace problem, implying that no change of state occurs, and there is no clue in the Euler-Cauchy approach that it was ever considered as one. The Euler-Cauchy theory of stress is incompatible with the potential theory and with the nature of the problem; it is therefore wrong. The key point in the understanding of elasticity is the elastic potential.