We introduce the smoothed density field δ δ M ( x → → ) , given by δ δ ( x → → ) averaged over a sphere with center x → → and mass M contained inside (i.e., δ δ is convolved with a top-hat window function). The sphere radius is of order M ∼ ∼ ρ ρ ¯ ¯ R 3 . Then if δ δ M ( x → → ) ≥ ≥ δ δ c , a halo exists at x → → with mass at least M .