  The separability problem is formulated in terms of a characterization of a
single entanglement witness. More specifically, we show that any (in general
multipartite) state \varrho is separable if and only if a specially constructed
entanglement witness W_{\varrho} is weakly optimal, i.e., its expectation value
vanishes on at least one product vector. Interestingly, the witness can always
be chosen to be decomposable. Our result changes the conceptual aspect of the
separability problem and rises some questions about properties of positive
maps.
