An essay cannot be usefully defined. It is something, like eloquence generally, which is not practiced by rule, but by a synthesizing imitation of its masters, many as they are, bearing on a theme or problem and expressing observations chosen and collected by, or drawn from, the vivifying individuality of the writer. We recognize an essay because it is part of that tradition.
Dr. Johnson furnished us with, "A loose undigested sally of the mind"—this from a lexicographer, this from a critic, whose language includes Bacon's plain-set diamonds! We call many compositions essays only for their length—in the hands of some the essay is merely a unit of composition arranged from paragraphs, just as paragraphs are arranged from sentences. Or it is the sump of belles-lettres, where everything ends up which has no where else to go. As these definitions are true, they are meaningless, two-legged without feathers.
An essay may be adumbrated by what it is not. It is not a treatise, it is not a tract, it is not a sermon. It does not begin from first principles, it deduces nothing; like conversation an essay takes interest, background, and receptivity for granted. The essay—as a genre, not an exercise—is not a tract; it does not line up points and facts to support them; nothing could be further from the spirit of the essayist than to expect to persuade, because it is the purpose of an essay, not to tell you what to think, but to expose where you have not thought. It is not a sermon: an essay has no reproach, but sympathy and commiseration for the vagaries of weak though willing humankind.
The very young are thus never good essayists: as they are untried, they think themselves sound; as they have hope, they think themselves destined; as they are pure, they think themselves incorruptible; as they are strong, they think themselves entitled. They need not be forcibly disabused; but until they have learned better, nothing they write, however clever, is really an essay.
An essay may be fine, patient work, or swift swashbuckling, but the tool is the same—to be honest; to omit nothing and exaggerate nothing. A secondhand opinion by cliché or party annuls the effort if it so begins, or derails it if it so ends. You bring to an essay only what is your own: not your personal philosophy, but the footnotes and exceptions and amplifications your experience and reflection have made to it; not your opinions, but what their trial, ventured in practice or maintained in conversation, has taught you of their place and value.