Ce que j'ai appris, je ne le sais plus. 
Le peu que je sais encore, je l'ai deviné.

"All that I've learned, I've forgotten. 
The little that I still know, I've guessed."
Camfort, Maximes et pensées, 1795. (posthumous) 

"The amnesia is real, whereas what is forgotten has lost its reality." (Winnicott, p.22)

Are all memories permanently stored somewhere in the mind, so that details we cannot remember at a particular time could eventually be recovered with the right technique? Or are some experiences permanently lost from memory

Who coined the term Active Forgetfulness? 
Was it Nietzsche? I forget. 

In the Geneaology of Morals, Nietzche pointed out that If I always remember, I cannot "have done" with anything. "so that it will be immediately obvious why there could be no present without forgetfulness." In "Literary History and Literary Modernity," Paul de Man follows the Nietzchean pattern when he fastens upon a particular type of forgetting as the core experience of modernity.

For Nietzche, one way to ward off forgetfulness is to lead an archetypal life, in which the past and the future are identical. 

Sigmund Freud required a theory of forgetting in his early clinical work on hysterics. He found that his patients did not have certain memories at their disposition, although these were perfectly vivid once they had been recalled: "...It was a question of things which the patient wished to forget, and therefore intentionally repressed from his conscious thought and inhibited and suppressed." ("Studies on Hysteria" 1895)

Kurt Goldstein, who rejects the idea of the unconscious, emphasises the continual formation of new patterns in the organism, in which the effects of former reactions have not been "forgotten" through repression, but cannot be remembered because they are no longer part of the attitudes of later life. 

Oliver Sachs talks about how quickly and imperceptibly one forgets motor abilities. "How could one know one had shrunk, if one's frame of reference had itself shrunk?" (A Leg to Stand On, p. 129) He describes the experience of "scotoma" as itself forgotten, by the person himself, the physician and his listeners, by history and culture, what Orwell called a "memory hole."

Langdon Winner describes technology as a "license to forget." (Autonomous Technology, p. 315) An achievement, once secured, can be safely forgotten and put behind one. We also "forget" the transformations of the world that a mediating technology entails as its becomes "transparent" (see tool)

"The process by which human beings get accustomed to new technical means that initially evoked mistrust and fear can be characterized as a process of repression of fear, or, more neutrally, as a diminution of fear." (Schivelbusch, Railway Journey, p.161) This forgetfulness is aided by the elimination or obfuscation of the anxiety-provoking manifestations, eg: vibrations, jolts.