  It is shown that there exists a duality among fields. If a field is dual to
another field, the solution of the field can be obtained from the dual field by
the duality transformation. We give a general result on the dual fields.
Different fields may have different numbers of dual fields, e.g., the free
field and the $\phi^{4}$-field are self-dual, the $\phi^{n}$-field has one dual
field, a field with an $n$-term polynomial potential has $n+1$ dual fields, and
a field with a nonpolynomial potential may have infinite number of dual fields.
All fields which are dual to each other form a duality family. This implies
that the field can be classified in the sense of duality, or, the duality
family defines a duality class. Based on the duality relation, we can construct
a high-efficiency approach for seeking the solution of field equations: solving
one field in the duality family, all solutions of other fields in the family
are obtained immediately by the duality transformation. As examples, we
consider some $\phi^{n}$-fields, general polynomial-potential fields, and the
sine-Gordon field.

General Physics