When we encounter the truth, it has a distinct and peculiar feel. It is not as though we assign that qualia to some of our notions, and not to others. We rather discover that the truth feels the way that it feels, in just the way that we find that oranges are orange and fire is hot. We don’t decide that an idea is true, we understand its truth. Its truth does not originate in us, but is rather borne in upon us forcefully. Nor once we have intuited the truth of a notion is it at all possible for us to delete or controvert that intuition (except insofar as we subsequently refine our understanding of the idea in question). Truth is not in the eye of its beholder; the eye of the intellect beholds it, not as an invention of its own, but as an objective reality.
The intuition of truth is given in our nature, rather as our intuitions of color and heat are given to us. Our nature is such as to conform our minds to the character of reality: to color, to heat, to truth. This, not only on account of the biological requirement of adaptive fitness, but because our nature is itself an instance of reality. Our first discovery of the character of reality then is given with our discovery of our own being.
An innate knowledge of the gods is coexistent with us, and this knowledge is superior to all judgement and deliberate choice, and subsists prior to reason and demonstration.
On the Mysteries of the Egyptians, Chaldeans & Assyrians
It is in virtue of that prior knowledge, given as implicate in the basic structure of our own nature, that the truths discovered by ratiocination are recognized as such. We see for example that the Law of Noncontradiction must be true in the light of that knowledge of our very own selves. If the Law of Noncontradiction were false, we simply could not be. So its truth is implicit in our actuality. We can see its truth when we contemplate our actuality. So likewise for all the necessary truths, such as those of logic.
Only knowing those truths as true could we then proceed to evaluate the validity of any argument.
So the primordial knowledge of First Things that Iamblichus notices is the forecondition of ratiocination.