  We present a general criterion for entanglement of N indistinguishable
particles decomposed into arbitrary s subsystems based on the unambiguous
measurability of correlation. Our argument provides a unified viewpoint on the
entanglement of indistinguishable particles, which is still unsettled despite
various proposals made mainly for the s = 2 case. Even though entanglement is
defined only with reference to the measurement setup, we find that the
so-called i.i.d. states form a special class of bosonic states which are
universally separable.
