  We study the origin of the resonances associated with pole singularities of
the scattering amplitude in the chiral unitary approach. We propose a "natural
renormalization" scheme using the low-energy interaction and the general
principle of the scattering theory. We develop a method to distinguish
dynamically generated resonances from genuine quark states
[Castillejo-Dalitz-Dyson (CDD) poles] using the natural renormalization scheme
and phenomenological fitting. Analyzing physical meson-baryon scatterings, we
find that the Lambda(1405) resonance is largely dominated by the meson-baryon
molecule component. In contrast, the N(1535) resonance requires a sizable CDD
pole contribution, while the effect of the meson-baryon dynamics is also
important.
