Essential properties are properties of an object or idea that that thing must have, or it would not be that thing.

For example, an essential property of a chair might be that you can sit on it. An essential property of a triangle is that it has three sides.

Some things don't have definite (or at least simple) essential properties. For example, most of us don't know what would be an essential property of a cat.

Compare to Accidental properties.

Baffo--that depends on whether you are using traditional Analytic Philosophy or Revisionary Philosophy. Analytically, you can shoot holes in any definition of chair you might come up with, but with revisionary philosophy a hole wont hurt anything. You use whatever the most useful definition of a chair is. (You could still modify that concept of a 'chair' with 'broken', of course.)

Illinois Avenue, Tennessee Avenue, and New York Avenue are often considered essential properties, as are the four railroads, Park Place and Boardwalk. Except for those last two, they are the most probable squares for any player to land on during a typical game of "Monopoly". Boardwalk comes in 18th, and Park Place at 33rd, but these squares usually make all the difference when the end of the game is nigh.

This concept is plagued with possible paradoxes.

Trying to define a chair, for example, is a serious nightmare. The proposed definition of "that which is fit for sittin on" fails with a broken chair.
Also, consider a chair whose seat has been punched out: you can't sit on it, but wouldn't you call it a chair ?

Essential and accidental properties are very much an Aristotle concept, and they fail in the face of modern logics.

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