In this case, I think may be remarking on the distinction between human beings and animals. That is, the rational faculty we possess enables us to, if we choose the route of Intellect, to raise ourselves above the merely irrational world of sense that otherwise makes up our existence in the physical world. If we choose instead the route of pure sensation and unreason, then we sink back to the level of animals. That's my conjecture, anyway.

Iamblichus does indeed get into the various classes of spiritual beings, but I suspect Sallustius is commenting on something different here.