Knowledge is like a sphere: the greater its volume, the larger its contact with the unknown. - Blaise Pascal
Consider the following idea: “It was possible as recently as three hundred years ago for one highly learned individual to know everything worth knowing. By the 1940s, it was possible for an individual to know an entire field, such as psychology. Today, the knowledge explosion makes it impossible for one person to master even a significant fraction of one small area of one discipline.” 
Now, if one understands ignorance to be both (a) an individual (rather than collective) phenomenon, and (b) measured according to the difference between what humanity as a collective whole knows versus what the individual knows, then it seems hard to deny that ignorance is rapidly growing. The reason is, basically, because as collective knowledge grows exponentially (or something close to that), the cognitive resources of the individual remain fixed and finite.
But some thinkers have argued that the same thing is happening with collective ignorance. On his blog The Technium, Kevin Kelly defines “ignorance” as the numerical difference between the questions that we, the collective whole, have posed versus the answers that we have provided to those questions. The idea here is that with each new answer comes two or more new questions. Kelly says:
Thus even though our knowledge is expanding exponentially, our questions are expanding exponentially faster. And as mathematicians will tell you, the widening gap between two exponential curves is itself an exponential curve. That gap between questions and answers is our ignorance, and it is growing [exponentially]. In other words, science is a method that chiefly expands our ignorance rather than our knowledge.
If Kelly is right about science being a more powerful engine of ignorance than knowledge, then it seems to follow that cognitive enhancement technologies would only further increase our ignorance of the universe. This is because a community of cognitively-enhanced scientists would presumably be far more productive and efficient than a community of unenhanced scientists, thus accelerating the rate the scientific “progress.” But could this be right? Might cognitive enhancement technologies actually make us dumber?
Kelly’s conclusion seems to be wrong. His claims confuse two distinct things: knowing that one doesn’t know, on the one hand, and not knowing that one doesn’t know, on the other. Of course, our ancestors from the Pleistocene knew nothing about “dark matter,” and neither do we. But our “epistemic” (or relating-to-knowledge) situation is importantly different than theirs; it has a positive aspect, since knowing that we don’t know is still knowing something!
One can think of this in Rumsfeldian terms: science is an enterprise primarily aimed at converting “unknown unknowns” to “known unknowns” to “known knowns.”  And while the first possibility has no positive status as knowledge, the second two surely do. Thus, even if all modern science had were questions, we’d still be better off – “epistemically” speaking – that our pre-scientific ancestors.
Okay, so cognitive enhancement technologies would not increase collective ignorance, at least not if defined as the numerical difference between questions that we could but haven’t yet asked (“unknown knowns”) and questions that we have asked and either answered or not (“known unknowns” and “known knowns”). 
But there is yet another category of questions that might be off limits to us in principle, questions that we humans could not possibly ask because they involve concepts beyond our ken. Following the Rumsfeldian nomenclature above, we might call these “unknowable unknowns,” i.e., we humans both don’t and can’t know about them. In contemporary philosophy (my primary area of study), some philosophers have termed questions of this sort mysteries. Unlike mere problems, they argue, questions that fall within this special category are beyond our “epistemic” reach no matter what, just like learning how to do arithmetic is beyond the reach of a monkey, or mouse, or cricket no matter what.
What I find especially exciting about cognitive enhancement technologies is the possibility of redefining the boundary between mysteries and problems. The most promising way of doing this would be through a qualitative modification of the human mind, although a quantitative one might do it too. Such modification would (possibly) make accessible to “us” (that is, to our posthuman progeny) important concepts for understanding the universe that are currently beyond our cognitive limits.
To recap, so far we have shown how cognitive enhancements could decrease collective ignorance. But might they actually eliminate it completely? Consider the following possibility: if everything that exists in the universe is physical, it stands to reason that there is a complete description, or final theory, of the universe. In technical terms, this position that only physical things exist is called “physicalism,” and it is by far the most popular view in contemporary philosophy and science. 
The problem here is that the second half of the above claim is ambiguous: it all depends on how one defines a theory. (Another issue relating to definition!) For example, if one defines a theory as “something that can be stated in a human language,” then physicalism might not entail that there is a final theory. This is because, once again, the final theory might involve concepts or require us to answer questions that we mere humans cannot grab hold of.
But if cognitive enhancements succeed in redefining the boundary between mysteries and problems, then we (that is, our posthuman progeny) might actually be able to comprehend such a final theory. In other words, by qualitatively augmenting our cognitive capacities in the right way, we could gain the ability to grasp the currently ungraspable concepts necessary to make sense of a complete description of the cosmos. And consequently, cognitive enhancements would not merely decrease but completely eliminate our collective ignorance, despite what Kelly would (have to) argue, given his account of ignorance. 
Good news for all the knowledge lovers out there!
1. Quoted from page 22 of Gregg Jacobs’ book The Ancestral Mind.
2. “Known unknowns” might be considered a type of learned ignorance, a concept developed by apophatic theologians like Nicholas of Kues. See this link for a video of Rumsfeld using this terminology.
3. This assumes that the number of questions we could but have not yet asked is finite. Obviously, if the number if infinite, then no increase in the number of questions asked/answered would change the ratio referred to here. See below for why we might expect the number of questions to be finite.
4. For an interesting discussion on the subject, see the Stanford Encyclopedia of Philosophy entry at this link.
5. Cognitive enhancements would also, of course, have implications for individual ignorance. Maybe it would be the case that individual posthumans could know everything the collective whole knows. Thus, cognitive enhancements could possibly eliminate individual ignorance too.