The full quote goes like this "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. And if one looks throughout the history of our country and other free countries, it is the latter category that tend to be the difficult one" (https://en.wikipedia.org/wiki/There_are_known_knowns).
As the years have passed, I am struck more and more at how correct Donald Rumsfeld was It is the unknown unknowns that really make a mess of our lives. "If only I had known... I am sorry, I never realised... " are the epitaphs to many a venture, project or relationship.
One day out of the blue, a wife/husband/partner walks into the bedroom and says "that's it, I don't love you anymore, we are finished" and when the other party has managed to recover his/her/its (gender neutral pronoun) composure to respond with a "whaa," then the immortal words are spoken "I had no idea".
Change the setting to the business world. Imagine that you are a talented young adult. You have just finished your apprenticeship to become a Scrivener, borrowed money from your parents to set up your writing shop, you open for business and then one Mr Caxton opens his printing press next door. You are obsolete, your skills are no longer needed. If you had known, would you have invested all that time and effort in becoming proficient in penmanship? Of course not. But how could you possibly know?
In the real world, it is not realistic to think that a human being can be all-wise and all-knowing. In a failing relationship it is another matter, your aggrieved other half, holds you responsible for every failing that happened both in the relationship and in the history of the universe since the big bang. "You SHOULD have known what was going to happen", is a common stick used by the unhappy partner to beat up the hapless other. As an English teacher, I frequently work with modal verbs, the ones that resonate most with me are: could have, should have and would have - very useful for mental self-flagellation.
So how do we make decisions? Do we base them on our accumulated knowledge and experience? Captain Smith of the Titanic was an experienced and knowledgeable trans-Atlantic Captain. Will we always have to be second guessing ourselves and others? On what basis do we trust our knowledge?
In this era of "Alternate Facts", how do we know what we know is true? Do we trust the news? How much do you believe the picture of the world that is presented by Fox News or Russia Today? What about celebrities who Tweet? Julian Assange used to be a personal hero of mine. Why? Because I liked the image of him that was portrayed on the websites I used to look at. Then I discovered that St Julian was only human after all and it seems that he was taking money (quite a lot of money) from a political group to tweet propaganda (https://casoaislado.com/la-generalitat-pago-a-julian-assange-para-que-defendiera-la-independencia-a-traves-de-twitter/). So another one bites the dust. Add him to the ever lengthening list of people who I used to like/trust. People like BIll Clinton and Barack Obama. Now it appears that Assange is to truth as Clinton is to marital fidelity and Obama to world peace (how many drone stirke deaths were authourised by the Nobel Peace Prize winner?).
So who or what can we trust? The mighty dollar? Not if you know anything about the size of the US national debt. Bitcoin? It has all the hallmarks of a bubble about to explode - do the expressions "Dotcom" and "Subprime bubbles" ring a bell? Going back to the dollar for a moment, the official motto of the United States "In God we trust" appears on its currency. Given that many centuries of philosophical and metaphysical study have never produced any solid evidence of the existence of a supernatural being (I am talking about fact based evidence and not faith based), even though it would be a great personal consolation if there was, I can only say that in this case, I know what I don't know and just like the character Jon Snow in the TV series Game of Thrones, I know nothing.
Instead of going off on a tangent inspired by Immanuel Kant's theories of the knowable and unknowable noumenon, I will finish up by saying the as I get older and older, I trust less and less this world where as Robbie Burns puts it:
"But, Mousie, thou art no thy lane [you aren't alone]
In proving foresight may be vain:
The best laid schemes o' mice an' men
Gang aft a-gley, [often go awry]
An' lea'e us nought but grief an' pain,
For promised joy."
As I get older and older, I agree more and more with the sentiments attributed to Mark Twain:
"The more I learn about people, the more I like my dog"
You can keep your Dollar and Bitcoin, for me it's, "in dog I trust".