At a press conference in 2002, US Secretary of Defense Donald Rumsfeld famously explained foreign policy: “There are known knowns; there are things we know we know. We also know there are known unknowns; that is to say, we know there are some things we do not know. But there are also unknown unknowns, the ones we don't know we don't know."
The same can be said of science.
The textbooks are full of known knowns. We are quite confident affirming the circulation of the blood, the distance to the Sun, the common descent of animals and plants. I sit here in the college library within a few steps of shelf after shelf of known knowns, the accumulated body of reliable knowledge -- the Qs, in our Library of Congress classification system -- upon which is based modern "civilization" -- the Rs, Ss, Ts, Us and Vs, medicine, agriculture, technology, and military and naval science.
To be sure, it is a characteristic of science that the known knowns are not etched in stone. Even the most confidently affirmed paradigms are written in pencil, ready to be erased or edited if necessity requires. That doesn't happen often. By and large, the known knowns accumulate at an ever-increasing rate. It is impossible for even the most accomplished scientist to keep up, except, perhaps, within a narrow specialty.
Then there are known unknowns. The nature of dark energy, for example. The mechanisms of memory and consciousness. The origin of life on Earth. Questions like these attract the curiosity of the most talented researchers. Faced with such questions, we say "I don't know yet, but I'd like to find out."
I have often used here the metaphor of knowledge as an island in a sea of mystery. The island is the known knowns. The shoreline is where we encounter the known unknowns. And then there is whatever is over the far horizon -- the unknown unknowns.
The known knowns are -- or should be -- a source of pride for our species. The known unknowns spark our curiosity. The unknown unknowns keep us undogmatic.
Or should keep us undogmatic. On the floor below me here in the college library are shelf after shelf of books -- the Bs, theology and religion -- that drag the unknown unknowns kicking and screaming into what is purported to be the known knowns. Much of sectarian violence was and is caused by dressing up unknowns unknowns as known knowns.
Are there unknown unknowns that are unknowable? Who knows? My guess is yes. We are finite creatures living (even as a species) for a finite time in a universe that may be infinite and eternal. Which should be reason enough to live lives of quiet, tolerant, agnostic reverence.
(These thoughts were inspired by a review in Nature by Michael Shermer of a book by Stuart Firestein.)