“Some speak of things we know, as new;
And you, of things unknown as things forgot.”

We know to tell many fictions like to truths, and we know, when we will, to speak what is true.
We know how to tell many lies that pass for truth, and we know, when we wish, to tell the truth itself.
Source: The Theogony (c. 700 BC), lines 27–28. Variant translations:

Department of Defense news briefing (12 February 2002)
Variant:
Now what is the message there? The message is that there are no "knowns." There are things we know that we know. There are known unknowns. That is to say there are things that we now know we don't know. But there are also unknown unknowns. There are things we do not know we don't know. So when we do the best we can and we pull all this information together, and we then say well that's basically what we see as the situation, that is really only the known knowns and the known unknowns. And each year, we discover a few more of those unknown unknowns.
:It sounds like a riddle. It isn't a riddle. It is a very serious, important matter.
:There's another way to phrase that and that is that the absence of evidence is not evidence of absence. It is basically saying the same thing in a different way. Simply because you do not have evidence that something exists does not mean that you have evidence that it doesn't exist. And yet almost always, when we make our threat assessments, when we look at the world, we end up basing it on the first two pieces of that puzzle, rather than all three.
:* Extending on his earlier comments in a press conference at NATO Headquarters, Brussels, Belgium (6 June 2002) http://www.nato.int/docu/speech/2002/s020606g.htm
2000s

Professor Van Helsing to Dr. Seward
Dracula (1897)
Context: You reason well, and your wit is bold, but you are too prejudiced. You do not let your eyes see nor your ears hear, and that which is outside your daily life is not of account to you. Do you not think that there are things which you cannot understand, and yet which are, that some people see things that others cannot? But there are things old and new which must not be contemplated by men's eyes, because they know, or think they know, some things which other men have told them. Ah, it is the fault of our science that it wants to explain all, and if it explain not, then it says there is nothing to explain. But yet we see around us every day the growth of new beliefs, which think themselves new, and which are yet but the old, which pretend to be young, like the fine ladies at the opera.

Quote in a letter to Sherwood Anderson, October 1923; as quoted in Georgia O'Keeffe: A Life, Roxana Robinson, University Press of New England, 1999
1917 - 1929
Context: I have been thinking of what you say about form... I feel that a real living form is the natural result of the individual’s effort to create the living thing out of the adventure of his spirit into the unknown.... and from that experience comes the desire to make the unknown known. By unknown I mean the thing that means so much to the person that he want to put it down - clarify something he feels but does not clearly understand... Making the unknown known.... if you stop to think of form as form you are lost.

The Wit and Wisdom of Grace Hopper (1987)
Context: You manage things, you lead people. We went overboard on management and forgot about leadership. It might help if we ran the MBAs out of Washington.

When asked about the decriminalization of marijuana in Texas "DFW Norml Interview" https://www.dfwnorml.org/2012/12/dj-m-squared-interview by NORML (20 December 2012)

Source: Teen Idol

Aldous Huxley, using the term "the doors of perception" which originated with William Blake in The Marriage of Heaven and Hell. It is sometimes credited to Morrison because he cited it in interviews as the inspiration for the name The Doors and without always crediting Huxley as the source.
Misattributed
Variant: There are things known and there are things unknown, and in between are the doors of perception.
Source: Letters from Joe