This paper constructs a logic of soft constraints where the set of degrees of preference forms a partially ordered set. When the partially ordered set is a distributive lattice, this reduces to the idempotent semiring-based CSP approach, and the lattice operations can be used to define a sound and complete proof theory. For the general case, it is shown how sound and complete deduction can be performed by using a particular embedding of a partially ordered set in a distributive lattice.