“The moment you know you know you know.”

Vol. XI, p. 288
Posthumous publications, The Collected Works
Context: We know only fragmentarily this extraordinary thing called life; we have never looked at sorrow, except through the screen of escapes; we have never seen the beauty, the immensity of death, and we know it only through fear and sadness. There can be understanding of life, and of the significance and beauty of death, only when the mind on the instant perceives “what is”. You know, sirs, although we differentiate them, love, death, and sorrow are all the same; because, surely, love, death, and sorrow are the unknowable. The moment you know love, you have ceased to love. Love is beyond time; it has no beginning and no end, whereas knowledge has; and when you say, “I know what love is”, you don’t. You know only a sensation, a stimulus. You know the reaction to love, but that reaction is not love. In the same way, you don’t know what death is. You know only the reactions to death, and you will discover the full depth and significance of death only when the reactions have ceased.

Georges Duhamel in THE HEART'S DOMAIN (1919). As it was composed in French, the wording in English may vary in translation. Theodore Geisel / Dr. Seuss was born in 1904, and would have been about 15 years old at the time that it was published. The full text can be found at the link below: We do not know the true value of our moments until they have undergone the test of memory. Like the images the photographer plunges into a golden bath, our sentiments take on color; and only then, after that recoil and that trans-figuration, do we understand their real meaning and enjoy them in all their tranquil splendor.
Misattributed

Variant: Life will give you whatever experience is most helpful for the evolution of your consciousness. How do you know this is the experience you need? Because this is the experience you are having at the moment.
Source: A New Earth: Awakening to Your Life's Purpose

Source: Tears of a Tiger

Love – That’s All Cary Grant Ever Thinks About (1964)
Context: I used to hide behind the façade that was Cary Grant … I didn’t know if I were Archie Leach, or Cary Grant, and I wasn’t taking any chances. … Another thing I had to cure myself of was the desire for adulation, and the approbation of my fellow man. It started when I was a small boy and played football at school. If I did well they cheered me. If I fumbled I was booed. It became very important to me to be liked. It’s the same in the theater, the applause and the laughter give you courage and the excitement to go on. I thought it was absolutely necessary in order to be happy. Now I know how it can change, just like that. They can be applauding you one moment, and booing you the next. The thing to know is that you have done a good job, then it doesn’t hurt to be criticized. My press agent was very indignant over something written about me not too long ago. “Look,” I told him. “I’ve known this character for many years, and the faults he sees in me are really the faults in himself that he hates.”

Source: Murder in the Cathedral

Source: Reasons to Stay Alive