  A role of skew-symmetric differential forms in mathematical physics relates
to the fact that they reflect the properties of conservation laws. The closed
exterior forms correspond to the conservation laws for physical fields, whereas
the evolutionary forms correspond to the conservation laws for material media.
  Skew-symmetric differential forms can describe a conjugacy of any objects
(that correspond to the conservation laws). The closed exterior forms describe
conjugated objects. And the evolutionary forms, whose basis are deforming
manifolds, describe the process of conjugating objects and obtaining conjugated
objects. From the evolutionary forms the closed exterior forms are obtained.
  This shows that material media generate physical fields. The relation between
evolutionary and closed exterior forms discloses the relation between the
equations of mathematical physics and field theories. This explains the field
theory postulates.
  Conjugacy is possible if there is symmetry. Symmetries of closed exterior
forms, which are conditions of fulfilment of the conservation laws for physical
fields, are interior symmetries of field theories. And symmetries of dual forms
(due to the degrees of freedom of material media) are external symmetries of
the equations of field theories. This shows connection between internal and
external symmetries of field theories.
