from Susan Sontag’s introduction to The Best American Essays, 1992:
The word essay comes from the French essai, attempt—and many essayists, including the greatest of all, Montaigne, have insisted that the distinctive mark of the essay is its tentativeness, its disavowal of closed, systematic ways of thinking. Its most obvious trait, however, is assertiveness of one kind or another.
To read an essay properly, one must understand not only what it is arguing for but what it is arguing against. Reading the essays written by our contemporaries, we easily supply the context, the public argument, the opponent, explicit or implicit. The passage of a few decades can make this almost impossible.
Essays end up in books, but they start their life in magazines. (It’s hard to imagine a book of recent but previously unpublished essays.) The perennial comes now mainly in the guise of the topical and, in the short run, no literary form has as great and immediate an impact on contemporary readers. Many essays are discussed, debated, reacted to in a way that poets and writers of fiction can only envy.
The influential essayist is someone with an acute sense of what has not been (properly) talked about, what should be talked about (but differently). But what makes essays last is less their argument than the display of a complex mind and a distinctive prose voice.