We give a nonrigorous derivation of the nonlinear Boltzmann equation from the
Schrödinger evolution of interacting fermions. The argument is based mainly on
the assumption that a quasifree initial state satisfies a property called restricted
quasifreeness in the weak coupling limit at any later time. By definition, a state
is calledrestrictedquasifree if the four-point andthe eight-point functions of the
state factorize in the same manner as in a quasifree state.