The term "quiddity" derives from the Latin word quidditas, which was used by the medieval scholastics as a literal translation of the equivalent term in Aristotle's Greek to ti en einai (τὸ τί ἦν εἶναι) or "the what it was to be (a given thing)".
Quiddity describes properties that a particular substance (e.g. a person) shares with others of its kind. The question "what (quid) is it?" asks for a general description by way of commonality. This is quiddity or "whatness" (i.e., its "what it is"). Quiddity was often contrasted by the scholastic philosophers with the haecceity or "thisness" of an item, which was supposed to be a positive characteristic of an individual that caused it to be this individual, and no other. It is used in this sense in British poet George Herbert's poem, "Quiddity". Example: What is a "tree"? We can only see specific trees in the world around us - the category "tree" which includes all trees is a classification in our minds, not empirical, and not observable. The quiddity of a tree is the collection of characteristics which make it a tree. This is sometimes referred to as "treeness". This idea fell into disuse with the rise of empiricism, precisely because the essence of things, that which makes them what they are, does not correspond to any observables in the world around us. Nor can it be logically arrived at.