You live through time, that little piece of time that is yours, but that piece of time is not only your own life, it is the summing-up of all the other lives that are simultaneous with yours. It is, in other words, History, and what you are is an expression of History, and you do not live your life, but somehow, your life lives you, and you are, therefore, only what History does to you.

Robert Penn Warren (1905-1989) American poet, novelist, literary critic

Band of Angels, ch. 6 (1955)
    (Source)

This is sometimes cited to Warren's World Enough and Time (1950), but is not found there.

Often seen edited down:

You live through that little piece of time that is yours, but that piece of time is not only your own life, it is the summing-up of all the other lives that are simultaneous with yours. [...] What you are is an expression of History.

Note that the narrator continues:

That is what I have heard said, but we have to try to make sense of what we have lived, or what has lived us, and there are so many questions that cry for an answer, as children gather about your knee and cry for a sweetmeat. No, it would be better to change the comparison and say it is like children gathering about your knee to cry for a story, a bedtime story, and if you can tell the right story, then these children, then these questions, will sleep, and you can, too.