The essence of a particular thing that gives it its unique particularity; those qualities that make an individual this specific individual and not some other.
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Haecceity (from the Latin haecceitas, which translates as "thisness") is a term from medieval philosophy first coined by Duns Scotus (c 1265 - 1308) which denotes the discrete qualities, properties or characteristics of a thing which make it a particular thing. Haecceity is a person or object's "thisness".
