Evaluation
Out of John Duns Scotus's four arguments against the need for divine illumination, I believe the two that rely on inductive reasoning can be immediately disqualified. The problem of induction is that we can never know that our conclusions from it are True. They may be pragmatically true, but only God -- that is, something outside of the system -- could know the rules that are actually at work. Henry is right in saying that if we could grasp the rules on our own, then we could deduce everything else that results from them on our own as well. As it stands, we do not have access of the rules, so everything that we attempt to deduce later must be held with a shred of doubt.
Scoutus's second and fourth arguments are, therefore, nullified, since they cannot lead us to grasping true principles.
Scotus's third point is that we can say for sure that the eye is seeing a bright light because the "seeing" is internal to the eye. It may or may not be detecting something truly happening external to itself, but we can be sure that it is, indeed, seeing. This is a bit like saying that thought proves the existence of the thinker, except that it might be a little more logically sound. That is, Scoutus is saying that sights proves the existence of the perception of sight, which is bordering on circular reason. He is saying that we cannot deny the principle of sensation because it is self-evident.
My main problem with his reasoning in this step is that I have difficulty determining how to apply it as a criticism to what Henry of Ghent is saying. This may be more a result of the particular passages of Scotus that I used in writing my overview -- that is, this particular argument may not in fact be intended as a specific attack on Henry's essay on divine illumination.
The same critique I think may be applied to the last argument. In the first listed item, Scotus says that an example of a true principle that humans can grasp without any divine illumination would be something that is true by definition, such as that all bachelors are unmarried.
This is an example of a priori knowledge. Normally when talking about a priori knowledge, philosophers are making the point that we can know some things prior to experiencing them; Scotus is using it in the sense that we can know some things without divine illumination or explanation. That is, our natural reasoning capabilities are able to grasp some principles completely on their own, without fear of error.
I think this successfully forces a wedge into Henry's argument: while he's right in saying that there is a class of objects of cognition that require divine illumination at some level to truly and fully grasp, Scotus has put his finger on a second class of objects that are knowable a priori. We do not need divine illumination to know that a triangle must have exactly three angles, for example.
We would, however, need divine illumination in order to positively confirm such commonplace things as physics formulas. In this and any field of science, we have theories that work and have worked throughout all known time, but that does not prove them to be fact.