It has been observed that not all negative polarity items (NPIs) are licensed in the same negative contexts, and different NPIs may be licensed by different negative expressions. This shows that Ladusaw's (1979) downward entailment is not precise enough to account for the distributional patterns of NPIs (van der Wouden, 1997; van der Wouden and Zwarts, 1993; Zwarts, 1986, among others). One well-known attempt to deal with this issue is to divide negative expressions into several subtypes. Using boolean semantics, Zwarts (1986; 1998) distinguishes three kinds of downward entailing licensors and accounts for heterogeneous NPI-licensing conditions by means of the semantic strength of negative expressions. This approach has been adopted to define Korean negation (Nam, 1994; Chung, 1993; Chung, 1997; Hwang, 2009). In this paper, however, we argue that the boolean semantic approach for negation is not adequate in characterizing the properties of Korean negative expressions and explaining the contexts of licensing Korean NPIs.