ESSENTIAL, adj. Pertaining to the essence, or that which determines the distinctive character of a thing. People, who, because they do not know the English language, are driven to the unprofitable vocation of writing for American newspapers, commonly use this word in the sense of necessary, as
April rains are essential to June harvests.W.C. Bartlett
2010 Update: That thing inside a thing without which that thing would be an entirely other thing, or, in fact, not a thing at all but a philosophy.