Sartre explained all this in what turned out to be about 800 pages of virtually incomprehensible philosophical babble to all but a select few, called Being and Nothingness. Before I surrendered helplessly, realizing one hundred fifty or so pages into it that I had no earthly idea what he was talking about, I came away with one basic tenet, although given my severely handicapped understanding there's a high likelihood I got it all wrong.

It was this: All of us are always in the process of becoming. I think the example he gave, to illustrate the difference between being and becoming, was of a musician trying to master a difficult passage. Once the passage has been mastered, the musician instantly becomes infinitesimally better in ability beyond the ability required to execute the passage. Otherwise, the musician would become or be the passage instead of being or becoming the musician who mastered the passage, on his or her way to becoming the musician who masters an even more difficult passage. And At least, that's what I got out of it.

If anyone would care to explain the other 799 pages, I would be most grateful, as I tend to agree with the basic propositions of existentialism as a philosophical foundation for governing one's life.