Iamblichus has been explaining and correcting the assumptions of Porphyry throughout the work, taking Porphyry's letter apart piece by piece, much like you see on heated discussions on message boards today. It's an ancient tradition, these flame wars. Iamblichus was a bit more polite than most of us modern folks, but you can still feel the heat.
In Section III, he's addressing questions Porphyry raised about divination. I've been into divination lately, out of a frustrating lack of insight into what I'm supposed to be doing with my life to bring more instant solutions to my outstanding problems. What I've found is that I don't have nearly the grip on interpreting prophecy that I thought I did. I'm tackling Geomancy hard core though, so part of it might be an unfamiliarity with a new system. I did a tarot reading the other day that was truly magical.
But in Chapter XVIII, he turns to the role of Spirits, whether Divine, Angelic, or Daimonic in the role of divination:
ANOTHER contest, however, awaits us, not less than that in which we have been before engaged, and which you immediately announce, concerning the causes of divination, "whether a God, an angel, or a daemon, or some other power, is present in manifestations, or divinations, or certain other sacred energies." But our reply to your question is simply this, that it is not possible for any thing to be performed in a manner adapted to sacred concerns in divine works, without the presence of some one of the more excellent natures, as inspecting and giving completion to the sacred energy. And where the felicitous operations are perfect, sufficient to themselves, and unindigent, of these the Gods are the leaders. but where they are media, and in a small degree fall short (164) of the extremes, they have angels as the powers that perfect and unfold them into light. And it is the province of daemons to effect those operations which rank as the last. But the right performance of actions which are effected in a divine manner, is entirely to be ascribed to some one of the more excellent natures. For since it is not possible to speak rightly about the Gods without the Gods, much less can any one perform works which are of an equal dignity with divinity, and obtain the foreknowledge of every thing without [the inspiring influence of] the Gods.So depending on the type of divination you're doing, you'll be working with either a God, an Angel or a Daimon if you're going to be effective. The "energy" that makes the divination work comes from the presence of the spirit.
Most of our divination practices fall into the "media" type, using various forms of sortilege, using some form of divinatory device to see into the future. These, he explains, "have angels as the powers that perfect and unfold them into light."
Iamblichus separated the types of spirits that we deal with into classes of Gods, Angels, and Daimons. Heroes, too, but those were humans of divine descent, which magicians are aware of being. Gods are the intelligences of the Planets and the spheres above the 7 planetary spheres. In the Greek pantheon, the planets were considered to be the celestial representations of the Gods. Mars was Mars, Venus was Venus. When they worked with "Gods" in their rituals, they were working with the divine aspects of the planetary spheres, or those of the Fixed stars, or higher.
When Iamblichus talks about Angels, he's talking about the spirits that are a phase more dense than the Divine spirits. In my cosmology, his "Angels" are my "Spirits" of the planets. They are closer to matter and can speak through the influence of whatever physical objects we use in divination.
The Daimons he refers to are the terrestrial spirits closest to matter. These may include the Genii Loci, the natural spirits of the land around any given location. River spirits, tree spirits, and even local Heroes can be considered Genii Loci. The Daimons he refers to may also also include the Spirits of the Goetia, or even some aspects of the Enochian Angels.
So when performing a divination ritual, it's important to include the appropriate spirit for the job.
Reading this in Iamblichus, I remembered this isn't the only place I've read about conjuring the appropriate spirit before performing a divination lately. Jason Miller covers this detail in Sorcerer's Secrets. If I remember right, the little book of card descriptions that came with the Crowley Toth Deck included an invocation of a specific spirit before performing the reading too.