We can know how two things are related even if we do not know one of those things. For example, suppose we know that a certain bathtub is full of water. If we now see water spilling out of it, then we know that something is inside the bathtub displacing water, even though we do not know what that thing is. We can tell by the movements of a distant star that it is near something, even if we cannot identify that something. We can, in general, know that some event was caused by another, even though we do not know what the cause is.
Scientific enquiry is often the business of looking for whatever it is that is doing the causing.
Not being able to identify or know a thing does not deter us from saying exactly how that thing, whatever it is, is related to something else. In short, he is not entitled to the claim that the mind-body problem is insoluble on any of the interpretations of what is cognitively closed to us that we have examined. It seems that nothing McGinn says should lead us to believe that the solution to the mind-body problem is cognitively closed to us. References Flanagan, O. 1992. Consciousness Reconsidered .