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Donald Rumsfeld, former US Defence Secretary, said there are things we know we know, and things we know we don’t know. “But there are also unknown unknowns – the ones we don’t know we don’t know.”

The issue of how we know things keeps me awake at night, especially the idea that in some areas we don’t even know what the questions are, let alone the answers. Rumsfeld was echoing the famous remark of J B S Haldane in the 1920s: “My own suspicion is that the universe is not only queerer than we suppose, but queerer than we can suppose.”

I wish I’d done more philosophy, because the question of knowledge is one of its key themes (aka epistemology) down the centuries. What I do know is that the diverse figures of Rumsfeld and Haldane are not alone in their questioning.

The sense of ultimate mystery is, of course, part of religion. If we think we have all the answers through science (or will have them all pretty soon) then there is no motivation to think about anything else. Unfortunately, a lot of the popular presentation of science gives the impression that with just one more push we will understand everything. A notable exception is Brian Cox, who acknowledges that even with recent advances in physics there will always be other things over the horizon. Although I believe he is an atheist, he also shows a sense of wonder about things that is at least half way religious.

Hardcore sceptics will now cry “God of the Gaps”, but that is not what I advocate at all. There is a crucial Rumsfeldian distinction between the unanswered questions we know about (the known unknowns) and the sense of mystery concerning what lies beyond them (the unknown unknowns). The mystery is most obvious I think in quantum theory, closely followed by cosmology.

Once we grasp that mystery is not just about what we don’t know, but about what we cannot know through science, perhaps we can free our minds to other sorts of knowledge, not contrary to science but standing alongside it as an equal partner in the great human enterprise. Otherwise, we may find that as regards the major issues for our world, issues that become ever more challenging, we have no answers at all.